Home > Articles > Certification > Cisco Certification > CCNP Voice / CCVP

This chapter is from the book

Dial Plan Components

A dial plan is the central part of any telephony solution and defines how calls are routed and interconnected. A dial plan consists of various components, which can be used in various combinations. This section describes the components of a dial plan and how they are used on Cisco IOS gateways.

Defining Dial Plans

Although most people are not acquainted with dial plans by name, they use them daily. A dial plan describes the process of determining how many and which digits are necessary for call routing. If dialed digits match a defined pattern of numbers, the call can processed and forwarded.

Designing dial plans requires knowledge of the network topology, dialing patterns, and traffic routing requirements. There are no dynamic routing protocols for E.164 telephony addresses. VoIP dial plans are statically configured on gateway and gatekeeper platforms.

A dial plan consists of these components:

  • Endpoint addressing (numbering plan): Assigning directory numbers to all endpoints and applications (such as voice-mail systems, auto attendants, and conferencing systems) enables you to access internal and external destinations.
  • Call routing and path selection: Multiple paths can lead to the same destination. A secondary path can be selected when a primary path is not available. For example, a call can be transparently rerouted over the PSTN during an IP WAN failure.
  • Digit manipulation: Manipulation of numbers before routing a call might be required (for example, when a call is rerouted over the PSTN). This can occur before or after the routing decision.
  • Calling privileges: Different privileges can be assigned to various devices, granting or denying access to certain destinations. For example, lobby phones might reach only internal destinations, while executive phones could have unrestricted PSTN access.
  • Call coverage: You can create special groups of devices to manage incoming calls for a certain service according to different rules (top-down, circular hunt, longest idle, or broadcast). This also ensures that calls are not dropped without being answered.

While these dial plan components can be implemented using a Cisco Unified Communications Manager server, the focus in this book is on implementing these dial plan components on a Cisco IOS router acting as a call agent.

Dial Plan Implementation

Cisco IOS gateways, including Cisco Unified Communications Manager Express and Cisco Unified Survivable Remote Site Telephony (SRST), support all dial plan components. Table 4-6 provides an overview of the methods that Cisco IOS gateways use to implement dial plans.

Table 4-6. Dial Plan Implementation

Dial Plan Component

Cisco IOS Gateway

Endpoint addressing

POTS dial peers for FXS ports, ephone-dn, and voice register directory number

Call routing and path selection

Dial peers

Digit manipulation

voice translation profile, prefix, digit-strip, forward-digits, and num-exp

Calling privileges

Class of Restriction (COR) names and lists

Call coverage

Call hunt, hunt groups, call pickup, call waiting, call forwarding, overlaid directory numbers

Dial Plan Requirements

Figure 4-12 shows a typical dial plan scenario. Calls can be routed via either an IP WAN link or a PSTN link, and routing should work for inbound and outbound PSTN calls, intrasite calls, and intersite calls.

Figure 4-12

Figure 4-12 Dial Plan Requirements

The dial plan defines the rules that govern how a user reaches any destination. Definitions include the following:

  • Extension dialing: Determines how many digits must be dialed to reach an extension
  • Extension addressing: Determines how many digits are used to identify extensions
  • Dialing privileges: Allows or disallows certain types of calls
  • Path selection: Selects one path from several parallel paths
  • Automated selection of alternate paths in case of network congestion: For example, using a local carrier for international calls if the preferred international carrier is unavailable
  • Blocking of certain numbers: Prevents unwarranted high-cost calls
  • Transformation of the called-party number: Allows appropriate digits (that is, DNIS digits) to be presented to the PSTN or a call agent
  • Transformation of the calling-party number: Allows appropriate caller-ID information (that is, ANI information) to be presented to a called party

A dial plan suitable for an IP telephony system is not fundamentally different from a dial plan that is designed for a traditional telephony system. However, an IP-based system presents additional possibilities. In an IP environment, telephony users in separate sites can be included in one unified IP-based system. These additional possibilities presented by IP-based systems require you to think about dial plans in new ways.

Endpoint Addressing Considerations

Reachability of internal destinations is provided by assigning directory numbers to all endpoints (such as IP phones, fax machines, and analog phones) and applications (such as voice-mail systems, auto-attendants, and conferencing systems). An example of number assignment is provided in Figure 4-13.

Figure 4-13

Figure 4-13 Endpoint Addressing

The number of dialable extensions determines the quantity of digits needed to dial extensions. For example, a four-digit abbreviated dial plan cannot accommodate more than 10,000 extensions (from 0000 through 9999). If 0 and 9 are reserved as operator code and external access code, respectively, the number range is further reduced to 8000 (1000 through 8999). If direct inward dialing (DID) is enabled for PSTN calls, the DID numbers are mapped to internal directory numbers.

The most common issue with endpoint addressing is related to the mapping of internal extensions to available DID ranges assigned by the PSTN. When the DID range does not cover the entire internal address scope, an auto-attendant can be used to route calls between the PSTN and the internal network.

One of the biggest challenges when creating an endpoint addressing scheme for a multisite installation is to design a flexible and scalable dial plan that has no impact on the end user. The existing overlapping directory numbers present a typical issue that must be addressed.

Endpoint addressing is primarily managed by a call agent, such as Cisco Unified Communications Manager or Cisco Unified Communications Manager Express.

Call Routing and Path Selection

Call routing and path selection are the dial plan components that define where and how calls should be routed or interconnected. Call routing usually depends on the called number (that is, destination-based call routing is usually performed). This is similar to IP routing, which also relies on destination-based routing. Multiple paths to the same destination might exist, especially in multisite environments (for example, a path using an IP connection or a path using a PSTN connection). Path selection helps you decide which of the available paths should be used.

A voice gateway might be involved with call routing and path selection, depending on the protocol and design that is used. For example, an H.323 gateway will at least route the call between the call leg that points to the call handler and the call leg that points to the PSTN. When a Cisco IOS gateway performs call routing and path selection, the key components that are used are dial peers.

In Figure 4-14, if a user dials an extension number in another location (8-22-2001), the call should be sent over the IP WAN. If the WAN path is unavailable (due to network failure, insufficient bandwidth, or no response), the call should use the local PSTN gateway as a backup and send the call through the PSTN.

Figure 4-14

Figure 4-14 Path Selection Example

For PSTN-routed calls, digit manipulation should be configured on the gateway to transform the internal numbers to E.164 numbers that can be used in the PSTN.

PSTN Dial Plan Requirements

A PSTN dial plan has three key requirements:

  • Inbound call routing: Incoming calls from the PSTN must be routed correctly to their final destination, which might be a directly attached phone or endpoints that are managed by Cisco Unified Communications Manager or Cisco Unified Communications Manager Express. This inbound call routing also includes digit manipulation to ensure that an incoming called number matches the pattern expected by the final destination.
  • Outbound call routing: Outgoing calls to the PSTN must be routed to the voice interfaces of the gateway (for example, a T1/E1 or a Foreign Exchange Office [FXO] connection). As with inbound calls, outbound calls might also require digit manipulation to modify a called number according to PSTN requirements. This outbound call routing usually includes stripping of any PSTN access code that might be included in the original called number.
  • Correct PSTN calling-party number presentation: An often-neglected aspect is the correct calling number presentation for both inbound and outbound PSTN calls. The calling number for inbound PSTN calls is often left untouched, which might have a negative impact on the end-user experience. The calling number that is presented to the end user should include the PSTN access code and any other identifiers that are required by the PSTN to successfully place a call using that calling number (for example, using the missed calls directory).

Inbound PSTN Calls

Figure 4-15 shows how gateways manage inbound PSTN calls.

Figure 4-15

Figure 4-15 Inbound PSTN Calls

The site consists of a Cisco Unified Communications Manager Express system with endpoints registered to it, connected to the PSTN over a digital trunk. The DID range of the PSTN trunk is 2005552XXX, and phones use the extension range 1XXX. The processing of an inbound PSTN call occurs in these steps:

  1. A PSTN user places a call to 1-200-555-2001 (that is, an endpoint with internal extension 1001).
  2. The call setup message is received by the gateway with a called number of 200-555-2001.
  3. The gateway modifies the called number to 1001 and routes the call to the voice port that was created when a Cisco Unified IP Phone registered with Cisco Unified Communications Manager Express.
  4. The phone rings.

Figure 4-16 provides a description of the required number manipulation when a gateway receives an inbound PSTN call.

Figure 4-16

Figure 4-16 Numbers in Inbound PSTN Calls

Both the called and calling numbers must be transformed:

  • The called number can be converted from the public E.164 format to the internal number used for internal dialing. This transformation ensures that the call matches the outbound dial peer that is automatically created at endpoint registration. Directory numbers are commonly configured with their internal extensions.
  • The calling number must be presented to the callee in a way that allows callback. Because access codes are commonly used to reach external destinations, a calling number forwarded to the destination should include an access code. Optionally, some region-specific prefixes might have to be added, such as the long-distance prefix in the NANP region, 1.

Outbound PSTN Calls

Figure 4-17 shows the call flow for an outbound call.

Figure 4-17

Figure 4-17 Outbound PSTN Calls

The site consists of a Cisco Unified Communications Manager Express system with endpoints registered to it, connected to the PSTN over a digital trunk. The access code is 9. The processing of an outbound PSTN call occurs in these steps:

  1. A user places a call to 9-1-300-555-6001 from the phone with extension 1001.
  2. The gateway accepts the call and modifies the called number to 1-300-555-6001, stripping off the PSTN access code 9. The gateway also modifies the calling number to 200-555-2001 by prefixing the area code and local code and mapping the four-digit extension to the DID range.
  3. The gateway sends out a call setup message with the called number set to 1-300-555-6001 and the calling number set to 200-555-2001.
  4. The PSTN subscriber telephone at 300-555-6001 rings.

Figure 4-18 summarizes the requirements for number manipulation when a gateway processes an outbound PSTN call.

Figure 4-18

Figure 4-18 Numbers in Outbound PSTN Calls

Both the called and calling numbers must be transformed as follows:

  • The called number processing involves the stripping of the access code. Optionally, some region-specific prefixes might have to be added, such as the long-distance prefix in the NANP region, 1.
  • The calling number must be converted from the internal extension to the public E.164 format. If the outgoing calling number is not configured on the gateway, the telco operator sets the value to the subscriber number, but this setting might be inaccurate if a DID range is available. For example, the calling number for a call originating from 1002 would be set to 222-555-2000. Setting the calling number is considered a good practice and ensures proper callback functionality.

ISDN Dial Plan Requirements

The type of number (TON) or nature of address indicator (NAI) parameter indicates the scope of the address value, such as whether it is an international number (including the country code) a "national," or domestic number (without country code), and other formats such as "local" format (without an area code). It is relevant for E.164 (regular telephone) numbers.

The TON is carried in ISDN-based environments. Voice gateways must consider the TON when transforming the called and calling numbers for ISDN calls.

ISDN networks impose new number manipulation needs, in addition to the typical requirements for PSTN calls:

  • Correct PSTN inbound ANI presentation, depending on TON: Some ISDN networks present the inbound ANI as the shortest dialable number combined with the TON. This treatment of the ANI can be a potential problem, because simply prefixing the PSTN access code might not result in an ANI that can be called back. A potential problem can be solved by proper digit manipulation on gateways.
  • Correct PSTN outbound ANI presentation, depending on TON: Some ISDN networks and PBXs might expect a certain numbering plan and TON for both DNIS and ANI. Using incorrect flags might result in incomplete calls or an incorrect DNIS and ANI presentation. Digit manipulation can be used to solve these issues.

In Figure 4-19, three different calls are received at the voice gateway. The first call is received from the local area with a subscriber TON and a seven-digit number. This number only needs to be prefixed with access code 9. The second call, received with a national TON and ten digits, is modified by adding access code 9 and the long-distance number 1, all of which are required for placing calls back to the source of the call. The third call is received from oversees with an international TON. For this call, the access code 9 and 011 must be added to the received number, as a prefix to the country code.

Figure 4-19

Figure 4-19 Inbound ISDN Calls

Digit Manipulation

Digit manipulation is closely related to call routing and path selection. Digit manipulation is performed for inbound calls to achieve these goals:

  • Adjust the called-party number to match internally used patterns
  • Present the calling-party number as a dialable number

Digit manipulation is implemented for outbound calls to ensure the following:

  • Called number satisfies the internal or PSTN requirements
  • Calling number is dialable and provides callback if sufficient PSTN DID is available

Digit manipulation is covered in Chapter 5, "Implementing Dial Plans."

Calling Privileges

Calling privileges are equivalent to firewalls in networking. They define call permissions by specifying which users can dial given destinations. The two most common areas of deploying calling privileges are as follows:

  • Policy-defined rules to reach special endpoints. For example, manager extensions cannot be reached from a lobby phone.
  • Billing-related rules that are deployed to control telephony charges. Common examples include the blocking of costly service numbers and restricting international calls.

Calling privileges are referred to as a "Class of Service," but should not be confused with the Layer 2 class of service (CoS) that describes quality of service (QoS) treatment of traffic on Layer 2 switches.

Figure 4-20 illustrates the typical deployment of calling privileges. The internal endpoints are classified into three roles: executive, employee, and lobby. Each role has a set of dialable PSTN destinations that is associated with it. The executives can dial any PSTN number, the employees are allowed to dial any external numbers except international destinations, and the lobby phones permit the dialing of local numbers only.

Figure 4-20

Figure 4-20 Calling Privileges Example

The deployment of calling privileges is covered in Chapter 5.

Call Coverage

Call coverage features are used to ensure that all incoming calls to Cisco Unified Communications Manager Express are answered by someone, regardless of whether the called number is busy or does not answer.

Call coverage can be deployed for two different scopes:

  • Individual users: Features such as call waiting and call forwarding increase the chance of a call being answered by giving it another chance for a connection if the dialed user cannot manage the call.
  • User groups: Features such as call pickup, call hunt, hunt groups, and overlaid directory numbers provide different ways to distribute the incoming calls to multiple numbers and have them answered by available endpoints.

Call Coverage Features

Cisco voice gateways provide various call coverage features:

  • Call forwarding: Calls are automatically diverted to a designated number on busy, no answer, all calls, or only during night-service hours.
  • Call hunt: The system automatically searches for an available directory number from a matching group of directory numbers until the call is answered or the hunt is stopped.
  • Call pickup: Calls to unstaffed phones can be answered by other phone users using a softkey or by dialing a short code.
  • Call waiting: Calls to busy numbers are presented to phone users, giving them the option to answer or let them be forwarded.
  • Basic automatic call distribution (B-ACD): Calls to a pilot number are automatically answered by an interactive application that presents callers with a menu of choices before sending them to a queue for a hunt group.
  • Hunt groups: Calls are forwarded through a pool of agents until answered or sent to a final number.
  • Overlaid ephone-dn: Calls to several numbers can be answered by a single agent or multiple agents.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020