Additionally, ensure the CUCM SIP Trunk Security Profile or Device Pool settings on the phone allow TCP/UDP communication with the SRST gateway IP address.
In Cisco Unified Communications Manager (CUCM) environments, Survivable Remote Site Telephony (SRST) provides call processing redundancy. If the connection to the CUCM publisher/subscriber is lost, the local router at a remote branch takes over call processing for SCCP (Skinny) and SIP phones. voice register global mode srst
When a SIP phone is registered to CUCM, it has the SRST reference IP address configured in its device settings (typically the branch router's IP). Additionally, ensure the CUCM SIP Trunk Security Profile
Or in older MGCP/SCCP SRST configuration: voice register global mode srst