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.