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 distinctive ringing or DID (direct inward dialing), 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 meant for, and the fax machine answers its calls by recognizing its own ringing style.
In ISDN, the various B channels do not normally have separate addresses, but are usually on the same address. For this reason, it is the switch (and not the application or a person who has dialed the number) that assigns calls to these channels.