Cisco Unified Survivable Remote Site Telephony

SRST (Cisco Unified Survivable Remote Site Telephony) is a service on voice-capable Cisco ISR (Integrated services) routers that provides a local fallback for Cisco IP phones that are connected to CUCM (particularly over WAN) when the cluster is unreachable such as from an internet outage or VPN issue. It's most common in branch offices and remote sites, as CUCM clustering is not always possible, and is not designed for this purpose. There are two modes that SRST can be used in. Unified SRST Mode, used by default, can handle basic call signaling and trunks for voice calls. Enhanced SRST Mode enables video support, BLFs, Basic Automatic Call Distribution (B-ACD), Hunt Groups, cBarge, Privacy, and shared lines. SRST is licensed per endpoint with Enhanced SRST typically being more expensive. SRST is configured a lot like other ISR voice services such as Cisco Unified Communications Manager Express (CME), using call-manager-fallback. Historically, Cisco Unified SRST Manager was a provisioning server to automate configuation from the CUCM cluster to SRST and E-SRST servers. It was deprecated startiing with release 12.2.