PBX manufacturers often extend or amend the rules for internal ISDN communications, so the cause code returned may not be exactly the same as the international standard. 

The point to note in the example given, is that neither the ISDN equipment, nor the public network, nor the intended recipient, is the cause of the failed communication. Sending the same fax call via a direct ISDN line would prove this.

Bear in mind also that the whole scenario could be repeated at the remote end of the connection, with the supplier, the PBX, and the remote port, all needing to be aware of the Bearer Capability being employed.

See the Practical Applications page at the end of this section for more information on diagnosing this kind of problem.