I have a large IPT deployment (CUCM 6.1) with 600 IP phones and 400 analog devices. The analog devices mostly consist of Aastra analog phones hung off of 25 VG224s. All phones are configured as SCCP, including the Aastra analog end-points on the VG224 (running 12.4(20). Intermittently, when an Aastra analog phone user goes off-hook to answer a call, the call will disconnect. If the VG224 analog end-point is changed to MGCP (and the other analog end-point making the call is changed to MGCP), then the problem does NOT occur. I have not been able to verifiy whether it occurrs when an IP phone calls an analog Aastra phone.
Also, if the Aastra phone is swapped to another manufacturer (such as Cortelco), the problem also goes away. The problem is: we need features in SCCP, so changing 300 analog end-points to MGCP is not a solution; and swapping out 300 analog phones from Aastra - Cortleco (or whoever) is not a solution. There must be some kind of feature/timer, incompatibility???!!!
I've been playing with timers, upgraded VG224 code, swapped tip/ring......the problem sometimes happens after 1-5 calls, or 50-100 calls, but certainly averages around 1/20. The customer is quite, quite annoyed at this point.