Cisco Call Forward Unregistered
One of the problems with SRST is that the CUCM is unable to identify if a whole site has gone offline and that it may possibly be in SRST mode. In such a case, a user trying to call a phone at remote site gets either a busy signal or rolls to voicemail (or the call handling behavior depending on the configuration). How do you reach the remote site from the HQ or main site if the remote site is in SRST mode? The problem is solved by the Call Forwarding Unregistered (CFUR) feature of CUCM.
On each device line directory number (DN), CFUR is a call forwarding option that is engaged when a device is not registered. This is true in the following instances:
- The phone is unregistered, which implies the phone was registered earlier
- The phone is unknown, which implies the phone was never registered
- A user device profile is not logged in, which implies the user profile configured with CFUR for a physical phone or softphone that is not logged into extension mobility (and this is a segue to a later discussion of another potential use of CFUR).
When an active DN is unregistered, a call to this line is redirected based on the CFUR configuration. Figure 5-9 explains the CFUR process in conjunction with SRST scenario.
Figure 5-9 CFUR-Based Call Routing
The following steps explain the CFUR operation in conjunction with SRST:
- Step 1. The phone with DN 2003 goes off-hook and dials 1003 at the remote site. The CUCM dial plan comes to action associated with the digits dialed (digit analysis).
- Step 2. Because the remote site is in SRST mode, all phones at that site have the status of unregistered, which is one of the preliminary conditions for CFUR to begin.
- Step 3. The CUCM call processing node handling DN 2003’s call setup request passes the call decision process to the ForwardManager subprocess for a call routing decision. In turn, the ForwardManager subprocess determines that DN 1003 has a CFUR setting of 914087771003 (a valid E.164 number routable on the PSTN).
- Step 4. The call is routed over PSTN to the remote site, where the call is handled as a direct inward dialing (DID) call (assuming that the calling search space [CSS] configured for CFUR has valid public switched telephone network [PSTN] access). After digit manipulation by the remote site’s SRST/E-SRST router, a call setup is sent to DN 1003 that is registered with the SRST/E-SRST router as ephone, and the audio channel is established thereafter (following the normal call setup process).
Another potential use case for CFUR apart from use of CFUR in conjunction with SRST for availability is that CFUR can also be implemented when a user is leveraging a soft client. For example, if a mobile user has a laptop with a softphone (for instance, Cisco IP Communicator or Cisco Jabber) and the user shuts down the laptop or places it into standby/sleep mode, CFUR can forward calls destined for the unregistered softphone to the user’s cell phone. The user does not have to set up Call Forward All (CFA) manually before closing the softphone application. If the softphone is not registered, calls are forwarded to the user’s cell phone. This is another application of the CFUR feature that improves availability in CUCM deployments.