Survivable Remote Site Telephony Overview
Survivable Remote Site Telephony (SRST) is an optional feature for sites that depend on a Wide Area Network (WAN) connection to a Cisco Unified Communications Manager node. SRST references, which are configured in the Cisco Unified Communications Manager Administration interface, allow IP gateways to provide limited telephony service to IP phones at the remote site in the event of a WAN outage:
IP phones at the remote site can call each other
calls from the PSTN can reach the IP phones
calls from the IP phones can reach the external world through the PSTN
When phones at the remote site lose connectivity to all associated Cisco Unified Communications Manager nodes, the phones connect to the SRST reference IP gateway. The status line indication on the IP phone shows the phone has failed over to the backup SRST gateway. When the connection to Cisco Unified Communications Manager is restored, the IP phones reregister with Cisco Unified Communications Manager and full telephony services are restored.
SRST supports remote sites that may have a mix of SCCP and SIP endpoints in addition to PSTN gateway access.