Further investigation using wireshark shows that I was mistaken and in fact a 486 “Busy Here” response is received when the user answers the phone. It does look as if the Uverse router is changing the SIP messages in some way.
I have found several posts in other forums about similar problems with Uverse but as yet have not seen a solution posted.
The user does have an ATT Uverse phone as part of the package, perhaps this is causing the problem?