ask your provider, over here it is 4 digits.
you can run a ISDN trace on the system and see what is coming in....
The feature is activated in HiPath 3000/5000 Manager E by selecting the "Call Monitoring" tab under Maintenance. The Start button starts Call Monitoring, whereupon the data of all available ports in the system is read out.
If the trace session is over, you can start the ISDN Message Decoder (ISDN Tracer) and use it to convert the trace file into a readable format