In an effort to understand your issue, how is the connection made? Your first post said PRI direct from TWT, but if the IVR is in the mix, how is the connection made?
If the T1 is coming into the PBX, is the IVR connected using another "PRI" connection, or a Line Side T1, or an analog connection? It may not be an issue with the PBX, or it could be how the IVR thinks it is getting a signal.
Scott M.