Cisco Unified SIP SRST provides backup to an external SIP proxy server by providing basic registrar and redirect server or back-to-back user agent (B2BUA) services. These services are used by a SIP IP phone in the event of a WAN connection outage when the SIP phone is unable to communicate with its primary SIP proxy.
Cisco Unified SIP SRST can support SIP phones with standard RFC 3261 feature support locally and across SIP WAN networks. With Cisco Unified SIP SRST, SIP phones can place calls across SIP networks in the same way as SCCP phones.
Cisco Unified SIP SRST supports the following call combinations:
SIP phone to SIP phone
SIP phone to PSTN / router voice-port
SIP phone to Skinny Client Control Protocol (SCCP) phone
SIP phone to WAN VoIP using SIP
For more information, refer the following documents: