Survivable Remote Site Telephony (SRST) is a Cisco router IOS feature set that uses the Skinny protocol to provide call-handling support for the local IP phones if the WAN connection to a central site Call Manager fails. During a failure, if this feature is enabled, the Cisco IP phone displays a message to indicate that it is in Cisco SRST mode and its functionality is limited, mostly to just calling on the remote site network. The SRST feature also provides centralized call processing with telephony service backup including off-net calls to 911. The Cisco IP Phone 7960/7940 displays the message "CM Fallback Service Operating." The Cisco IP Phone 7910 displays the message "CM Fallback Service."
When the Cisco Call Manager is restored, the displayed message is removed and the full functionality of the Cisco IP phone is restored.
Depending on the configuration, the following features can be maintained in fallback mode:
(1) Re-homing of IP phones to a local router
(2) Local Extension-to-Extension calls
(3) Extension-to-PSTN calls
(4) Existing calls upon recovery
(5) Support for IP and POTs phones
(6) Speed dial and last-number redial
(7) Direct Outward Dial Calling (DOD) calling
(8) Direct Inward Dial Calling (DID) calling
(9) Caller ID and ANI support
etc. etc.
Hope this helps.
F C Wood
fc@wood.org
Houston, TX
