call-manager-fallback ip source-address ...
If you are currently , I can help you find the specific cause. Compare the differences between SIP SRST and SCCP SRST ? voice register global mode srst
By issuing the mode srst command, the router changes its behavior: call-manager-fallback ip source-address
Setting the mode to SRST on the branch router allows it to take over call processing automatically. This ensures that local users can still make internal calls, reach emergency services, and use PSTN gateways even when the primary "brain" of the phone system is unreachable. Key Functional Components reach emergency services
Or in older MGCP/SCCP SRST configuration:
However, based on the terminology, this appears to be a related to SRST (Survivable Remote Site Telephony) .