When configuring the PBX, external MSNs are assigned to internal MSNs. For every VoIP terminal device, a free internal MSN is linked to an external MSN.
External and internal calls from ISDN terminal devices into VoIP telephony
First, the ISDN terminal devices forward the desired destination number to the ISDN PBX when the call is being established. If the number is an internal number/MSN, then the PBX directs the call to the internal ISDN bus. The SIP terminal equipment connected to the router can only be reached by an internal call if the PBX knows the internal number for the VoIP user.
If your PBX is able to direct external numbers to the internal ISDN bus, then the ISDN terminal devices can also use the lines configured in the router, such as the SIP provider line, for outgoing external calls.
