Some installations support the assignment of more than one address to a single channel. On POTS lines, multiple addresses are made possible by various systems, such as DID (direct inward dialing) or distinctive ringing, which are extra-fee services provided by the telephone company.
Many large corporations use DID for incoming calls. Before a call is connected, its destination extension number is signaled to the PBX, which causes the extension to ring instead of the operator's phone. An example of distinctive ringing in a private home would be if the parents used one address, the children another, and a fax machine a third. Because only one line connects the house to the telephone network, all phones ring when a call appears, but the ring pattern is different depending on the number dialed by the calling party. With distinctive ringing, the people know who the incoming call is for, and the fax machine answers its calls by recognizing its own ringing style.
In ISDN, the various B channels might not have separate addresses. Because these B channels might be on the same address, it is the service provider (and not the application or a person who has dialed the number) that assigns calls to these channels.