Ralph, thanks for your reply, I'm running QSIG E1 and not T1 and would prefer ISO.
We are connected to an old Avaya system with voice mail on another MCD system.
The issue we have is when calling (Avaya) extensions with call forward on busy - this will send the call back-up the QSIG link - we receive a very large QSIG message (I have the this somewhere) that is not understood by MCD and results in an error message displayed in the maintenance window. Thinking the Avaya is including call forwarding reasons codes or similar that need disabling.