It might be opposite hangup rather then operator hangup, its a common error message, and ususally means there is an issue with the ISDN service (i had it here, my pri circuit died whilst i was on a call, and the display said opposite hangup when it had dropped).
Try restarting the system/ISDN card, as it may be a clocking issue.
I remember when the TEPRI-a's first came out there were clocking issues, and things like mobile and std numbers where the first ones to start failing.
Restart of the PRI card would fix it for a while then it would eventually come back, it took a few versions of firmware and a hardware mod to fix it, but i dont think this is going to be your issue.
