We have an install including 2 CUCM 5.1 servers (Pub and Sub), about 80 phones, and one voice gateway (with 1 PRI, and 1 T1 CAS circuit). All endpoints register with the Sub as their primary server with the Pub being the backup. Almost all incoming/outgoing calls go through the PRI. The VG is a 2851, running 12.4(15)T5, and is configured in MGCP mode. When doing our normal testing at cut-over time, we noticed that any established external calls got dropped when the Subscriber was taken down. All endpoints would register properly with the Pub, including the MGCP gateway and the PRI/T1 CAS ports, but active calls were not preserved. According to our Cisco SE, and some literature I've seen on CCO, this should work as advertised, but it's not. I believe I have seen this behavior on other installs as well, but thought that was normal until now.
I am attaching the "show run" of the 2851 gateway. If anybody has any ideas as to what might be the cause of this, I would appreciate your feedback.