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Overview
One of the big advantages of ISDN over an analogue service is the ability to assign multiple phone numbers to one line. In Europe (and other countries using the EuroISDN specification) this is implemented using Multiple Subscriber Numbering.
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According to the Euro-ISDN specification, the numbers do not have to be in sequence. Therefore, some providers will allocate non-contiguous MSN numbers like 810 3500, 810 3632, 810 4592, 810 7653 etc.
Incoming Calls: Directing calls to specific devices using MSN
Whenever you wish to receive calls from more than one type of remote device or service, you will need to tell the difference between these calls, so that, for instance, fax calls are answered by your fax software and not by your remote access software.
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You should not confuse this 'destination number' with the 'origination number' (the caller's number provided by the service, known as the 'Calling Line Identity', or 'CLI'). The caller's number can also be optionally provided on the incoming call, and some ISDN equipment can make use of this information (see below).
Outgoing Calls: Assigning MSN numbers to identify specific callers
The EuroISDN standard allows an outgoing call to present the originating number to the recipient by a feature known as Calling Line Identification, commonly abbreviated to CLI.
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