After upgrading from Cisco CallManager 3.3(2)c to CallManager 4.0(2), the user experiences these problem on the VG200 gateway running Cisco IOS Software Release 12.2(08)T, using Media Gateway Control Protocol (MGCP). To reproduce this issue, perform these steps:
Attach an analog phone to the VG200 and call another IP phone.
The IP phone rings, and the analog phone caller hears ring back tone.
When the IP phone answers the call, the call is dropped on IP phone, and the analog phone caller hears dial tone.
The analog phone caller can then dial another number, and events are repeated. (Refer to step 2).
There is an internal bug in Cisco IOS Software Release 12.2(08)T that affects Cisco CallManager 4.0 and MGCP voice gateways.
The bug is resolved in release 12.2T. However, VG200 cannot be upgraded to this version since it does not meet the memory requirements as described in Cisco bug ID CSCeg16700 - Upgrade of CCM from 3.x to 4.x VG200 requires 12.2T IOS.
For a workaround, configure the VG200 voice gateway using the H.323 protocol.