It sounds like you have a hardware detect configuration problem causing the disconnects. This is usually set in class 4 and 5 switches with E&M signaling. This will ensure calls properly disconnect when someone hangs up the phone (reversal ground, I’ think). Typically this is set with a timer to disconnect if caller/switch/PBX doesn’t send the hardware off hook/on hook signal (ground reversal?). Most switches can be set to “detect hardware answer- off”.

Make sure the switch is sending back a connect. If we don't get a connect after 3 minutes (if memory serves me correctly) we'll disconnect the call. If you can't find anything, go ahead and open up a TAC case; you'll need 'show ver', 'show run', 'debug voip ccapi' and 'debug vtsp all' OR 'debug vpm all' depending on the platform/version.