[VoiceOps] ATT Wireless Dropped Calls

This is a strange issue I just found out about, not sure how long it has been happening. Apparently any ATT Wireless calls that come into our system (an possibly just those that terminate into asterisk via sip(only complaints so far)) are dropped. Our switch is connected to ATT Wireless over ISUP/SS7 trunks and the calls are coming out as SIP. Asterisk gets a SIP INVITE from the switch, then responds with a 100 Trying and a 200 OK, The switch then sends an ACK and then a BYE. The ATT Wireless customer just hears beep beep beep call lost. Any ideas? It doesn't happen 100% of the time, but the vast majority. Also a Wait(1) in asterisk before answering doesn't seem to make any difference? Thanks for any ideas! -Eric