Core Issue
If the calls are answered within 20-25 seconds of ringout, the calls remain connected and have no issues. However, if not answered, the calls drop.
In the MGCP Delete Connection (DLCX) message, the reason code is 902, a loss of lower-layer connectivity.
The MGCP gateway gets a disconnect event from the lower (CCAPI) layer and sends a DLCX.
Resolution
In order to resolve the issue, add these two commands to the MGCP gateway:
mgcp rtp unreachable timeout 1000 action notify
no mgcp timer receive-rtcp
Ideally, this resolves the mid-call drop issue.