under what conditions are you getting that error. I was getting a similar error, when a user was complaining about not being able to dial a specific outbound number. The call would give and ISDN denial event, then just hang up.
I called the number from my cell and I got the "all circuits busy" message from the network. For some reason the AVAYA was not playing that message, just getting the denial event and hanging up.
RTMCKEE