My CUCM 7 system is connected to a 3rd party SIP based phone system. Calls can be placed and received fine via the SIP trunk created between the two. The phones on the SIP system can call Unity directly no problem by dialing the voicemail pilot number. If one of the phones calls to a phone on the CUCM side and it does not answer and the call goes to Unity, the user on the SIP side hears a fast busy. What would I be missing here? The SIP trunk is setup for G.711 since the remote phone system running SIP protocol cannot do G.729.