Cisco 5300 voice issue

Hey Everyone i was wonderign if you had any idea’s of this problem. Let me explain. We call voip to a 5300 gateway and the gateway is connected via ISDN to a NACT Switch. When the call comes into the NACT, it play’s a audio script to the caller. But sometimes we call and we see the connection to the 5300 but we do not hear the audio. we punch in numbers we see the digits but after 30 seconds it hangs up by itself. I have 2 cisco debugs 1 of a call that connects and finishes and 1 of a call that just dies with no air. I can not seem to find any differences. Any Suggestions??