Integrating ISDN terminal equipment into the router VoIP environment and the necessary steps for configuration depend upon the application at hand and, if applicable, upon the options available with a PBX. The main questions to be answered by the user are as follows:
- Can ISDN terminal devices telephone internally with SIP users?
- Are ISDN terminal devices available externally over SIP lines?
- Can ISDN terminal devices telephone externally over SIP lines?
If ISDN terminal equipment can be reached over an ISDN TE interface on the router, it is described as "upstream". From the perspective of the router, the ISDN terminal devices are on an external line. This ISDN terminal equipment is normally not classified as being for local users, and so no entries for ISDN users are necessary.
ISDN terminal equipment at an upstream ISDN PBX...
- can make internal calls to SIP users if the corresponding telephone numbers are configured as internal MSNs in the ISDN PBX.
- can receive internal calls from SIP users if the internal MSNs of the ISDN equipment are output to the ISDN line by the call-routing table, for example over a standard route.
- can only make calls over SIP lines if the PBX is able to output certain call numbers over its internal ISDN bus. Otherwise, all calls not matching with its internal MSNs would be forwarded by the ISDN PBX to the public telephone network.
- can only receive calls from an upstream SIP PBX if entered into the router as an ISDN user and registered as such with the SIP PBX.
If ISDN terminal equipment can be reached over an ISDN NT interface on the router, it is described as "downstream". For the router, this is then a local subscriber that can be reached via the list of registered users. As ISDN terminal devices cannot send domain information to register at the router, this must be entered as an ISDN user so that it can be made known to the VoIP system.
ISDN terminal equipment at a downstream ISDN PBX...
- can make internal calls to SIP users by entering the character for an outside line as required by the PBX and then dialing the SIP user's internal number. The PBX then forwards the call to the SIP user's internal number—without the outside-line access code—over its external ISDN bus to the router.
- can receive internal calls from SIP users as long as the entry for the ISDN user contains the correct assignment of the internal number to the appropriate MSN. The router takes a call to the ISDN user's internal number, translates it to the MSN, and outputs it to the allocated ISDN bus. The PBX receives the MSN as if it were an external call and forwards it to the corresponding ISDN terminal equipment.
- can conduct incoming and outgoing calls over SIP and ISDN just like SIP users. Again, the outside-line code may be necessary for outgoing calls.
