A real humdinger! Pretty simply, the conference unit had S-Bus ISDN lines from the PABX to the conference unit. The incoming PSTN line is a E1 (2Mb) line. The unit could establish outgoing calls to other units, but could not receive.
On testing the S-Bus lines and instigating a "loop" call on the port, the ISDN tester would have the call immediately released, "CAUSE 16". On replacing the BRI board and testing on another system, it beame clear that the fault was programming specific on the clients PABX, as the problem did not occur on other PABX's.
The resultant problem was that the S-Bus trunks belonged to the same Public Directory number group. Hence, any call sent to/from the port was prefixed with the client predigits. Placing all the S-Bus trunks in an unmodified - default - table, solved the problem.
See commands: [1030] per trunk, and [2220] on the BusinessPhone 250.
