Core Issue
This is working as designed (WAD).
At this point, MGCP PRI backhaul does not support call preservation when switching from MGCP to SRST or from SRST to MGCP.
Resolution
Media Gateway Control Protocol (MGCP) PRI backhaul does not support call preservation when transitioned from MGCP to Survivable Remote Site Telephony (SRST) or from SRST to MGCP. Refer to MGCP Gateway Fallback Transition to Default H.323 Session Application for more information.
Because the PRI is backhauled (transported) to Cisco CallManager in normal conditions, and when a gateway loses connectivity to the Cisco CallManager, the PRI Q.931 has to terminate locally on the IOS . Therefore, the gateway has to remove backhaul from the D-channel and reset the D-channel so that the active call is dropped.
When the gateway rehomes to Cisco CallManager, the gateway needs to backhaul the PRI to the Cisco CallManager. Also, when the gateway tries to configure backhaul to the Cisco CallManager, the D-channel needs to be reset and the active call is dropped. Therefore, MGCP PRI backhaul cannot support call presevation during fallback to the SRST or rehoming back to the Cisco CallManager.
To provide call preservation during SRST fallback and rehome, use H.323. Refer to the Configuring H.323 VoIP Call Preservation Enhancements for WAN Link Failures section of Configuring H.323 Gateways for more information.
