We are currently working on integration to a 3rd party PBX. The connection is via an ISDN PRI 'TIE' line utilizing NI2. We are the Network side and they are the User. The issue is our option 11 is utilizing Call-by-call routing. We have configured a TIE service route and the D-Channel is stable, we can 4- digit dial an extension on the other switch, we can assign a DID and pass this call directly though. However a problem arises when the other switch attempts to initiate a call to one of the Nortel extensions or through us to the CO. We disconnect the call every time with an ISDN Cause Code 16. We believe it is because we do not receive an NSF code from the other switch but can not be sure. This switch is unable to provide us with an NSF code either. Has anyone ever experienced this? If yes, what is the recommended work around?