The Cisco CME overlay feature provides a way to work around the physical button limits on your IP phones. Instead of a normal one-line-to-one-button mapping arrangement, you can map up to ten lines or ephone-dns to the same physical phone button. This allows you to use the same phone button to answer incoming calls on any of the up to ten ephone-dns associated with the button.
Earlier, you saw one usage (in Example 5-17) for overlay-dns, where it allowed access to multiple instances of ephone-dns that have the same telephone number. See Figure 5-2 earlier in this chapter.
The Purpose of an Overlay-dn
An overlay-dn associates or binds from two to ten ephone-dn IP phone lines onto a single IP phone button (even on single-line IP phones). You can use separate overlay-dn arrangements on each separate IP phone button. Each IP phone can use an independent set of ephone-dns for overlay for each of the phone's buttons.
An overlay-dn acts as a multiplexor. It dynamically selects the most appropriate ephone-dn to present on an IP phone button from within the configured overlay-dn set. When you receive incoming calls, the first ringing ephone-dn in the overlay set is presented. When you make an outgoing call, the first idle ephone-dn in the overlay set is selected.
You can configure the ephone-dns used in an overlay set as either single line or dual line. However, all the ephone-dns in the same overlay set must be of the same type (single or dual line).
Using Overlay to Overcome Phone Button Count Limits
The simplest use of overlay-dn is to overcome the limited number of physical buttons available on an IP phone. In a simple Key System case where you have four incoming PSTN trunk lines and four IP phones, you can make each line available on all phones simply by using one button per line. You can do this using a Cisco 7960 IP Phone, which has six buttons. With this arrangement, you can answer any of the four incoming lines on any of the four IP phones.
However, you cannot use this simple one-button-to-one-line mapping if you want to have ten incoming PSTN lines and ten IP phones (assuming a six-line Cisco 7960 IP Phone). There simply aren't enough line buttons to do this (unless you add a Cisco 7914 IP Phone Expansion Module to your Cisco 7960 IP Phones).
Example 5-30 shows how you can map ten incoming PSTN lines to a single button on an IP phone.
Example 5-30 Overlay-dn Configuration
router#show running-config ephone-dn 101 number 4085550101 no huntstop ephone-dn 102 number 4085550101 preference 1 no huntstop ephone-dn 103 number 4085550101 preference 2 no huntstop ephone-dn 104 number 4085550101 preference 3 no huntstop ephone-dn 105 number 4085550101 preference 4 no huntstop ephone-dn 106 number 4085550101 preference 5 no huntstop ephone-dn 107 number 4085550101 preference 6 no huntstop ephone-dn 108 number 4085550101 preference 7 no huntstop ephone-dn 109 number 4085550101 preference 8 no huntstop ephone-dn 110 number 4085550101 preference 9 ephone 1 mac-address 000d.1234.ecfd button 1o101,102,103,104,105,106,107,108,109,110 ephone 2 mac-address 000d.4321.a6b7 button 1o101,102,103,104,105,106,107,108,109,110 ephone 3 mac-address 000d.5678.b923 button 1o101,102,103,104,105,106,107,108,109,110
The key to this configuration is the o separator used in the button command in place of the normal : separator character. In this configuration example, the preference and huntstop commands are used to control the order of selection of the incoming lines (in preference order from 101 to 110).
This configuration allows any incoming call to be answered on any of the IP phones. The example shows only the first three IP phones. There is no specific limit on the number of phones that can share the lines as shown. But, in general, you want to limit the number of phones so as not to exceed the number of lines. If there are more phones than lines, some phones can't access any lines if they are all in use.
Using Overlay with Intercom
You can include ephone-dns configured for intercom within an overlay set. In general, you would do this only for one-way intercoms where the phone with the overlay intercom is not expected to initiate an outgoing intercom call. This allows you to attach an incoming-only intercom to an IP phone without using up one of the phone's buttons. If the intercom is used as incoming only, there is no need to assign a phone button to select the intercom. One example of incoming-only intercom is the many-to-one intercom case discussed earlier.
Overlays and Shared Lines
In the section "Using Shared Lines for Call Coverage," you saw how a call can be put on hold on one phone by pressing the hold softkey and then picked up by pressing the resume softkey on a second phone that shares the line.
This form of shared-line direct call pickup is unavailable in the case of lines shared by a phone in an overlay set. As soon as an ephone-dn is dynamically associated to a specific phone using an overlay, the ephone-dn is no longer accessible on other phones that share the ephone-dn using overlay. It is available on phones that directly share the line using a simple nonoverlay button assignment (for example, by using the : separator, as in button 1:tag).
You can still move calls between phones in this arrangement by using the pickup softkey, provided that you carefully number the ephone-dns within the overlay set so that they can be uniquely selected. Example 5-31 uses a simplified form of the previous configuration.
Example 5-31 Simplified Overlay-dn Configuration
router#show running-config ephone-dn 101 label 4085550101 number 101 secondary 4085550101 no huntstop ephone-dn 102 number 102 secondary 4085550101 preference 1 no huntstop ephone-dn 103 number 103 secondary 4085550101 preference 2 no huntstop ephone-dn 104 number 104 secondary 4085550101 preference 3 ephone 1 mac-address 000d.1234.ecfd button 1o101,102,103,104,105,106,107,108,109,110 ephone 2 mac-address 000d.4321.a6b7 button 1o101,102,103,104,105,106,107,108,109,110
In Example 5-31, you can see that the PSTN telephone number is moved to the ephone-dn's secondary number field. The primary number field is provisioned with a unique number 101, 102, 103, 104, and so on for each ephone-dn. This makes each ephone-dn in the overlay set uniquely identifiable.
The first ephone-dn in the overlay set also includes the label 4085550101 command. The label command overrides the normal line display behavior to prevent the phone from displaying the ephone-dn's primary number (in this case 101) and instead displays the desired PSTN number.
With this arrangement, consider an incoming call answered on ephone 1. When ephone 1 puts the call on hold, the phone display shows the ephone-dn primary number of the specific ephone-dn in use and that now has the call on hold. The phone display shows Hold . The call can then be accessed by another ephone (for example, ephone 2) by pressing the pickup softkey and entering the ephone-dn extension number displayed.
Another way to move the call from one phone to another in this arrangement is to use call park. (You'll read more about this shortly.)