Survivable Remote Site Telephony

Survivable Remote Site Telephony (SRST) is a Cisco Unified Communications Manager (CUCM) call processing backup mechanism that allows Cisco IP phones to register to a Cisco router. A Cisco router in SRST mode provides call setup and call teardown services similar to CUCM, but only when connectivity to the CUCM servers provided in the Cisco Call Manager group are not reachable. SRST technology is widely used in the centralized call processing model where branch sites do not have any local call processing capabilities. Cisco IP phones at branch locations register with CUCM over wide area network (WAN) links. All call setup and call teardown functions of the branch site Cisco phones are carried over the WAN links. Call setup and call teardown (call control) bandwidth requirements are benign (under 1kbps).

SRST router needs minimal configuration
Remote Site IOS router take over SCCP call processing for local IP Phones in case of WAN failure
Basic Call functions and features are preserved