When you have an ISDN connection, one link (with 2 ISDN channels) can get more phone numbers than one. Those numbers are called MSNs. You can program ISDN devices to listen to all or to specific MSN numbers.

For example, in Germany every ISDN subscriber get's 3 MSN numbers for free. So one could set one MSN for himself, one for his daughter and one for the FAX machine.