In a centralized Cisco Unified Communications Manager environment, when IP phones lose connectivity to Cisco Unified Communications Manager because the WAN is down or Cisco Unified Communication Manager is unreachable, IP phones in remote branch offices lose call-processing capabilities. The SRST feature provides basic IP telephony backup services so that IP phones can fall back to the router in a local branch office when losing the connectivity to the Cisco Unified Communications Manager. The SRST feature takes advantage of existing Cisco IOS? Software features to provide basic telephony services such as off-net calls to 911, calls within a branch office, or calls between branch offices through the public switched telephone network (PSTN). The application is ideal for enterprise organizations looking to cost-effectively deploy IP telephony in their branch office location.
This URL should help you:
http://www.cisco.com/en/US/products/sw/voicesw/ps2169/products_qanda_item0900aecd8028d113.shtml
