Unlike line devices and addresses, calls are dynamic. A call represents a connection between two (or more) addresses. The originating address (the caller) is the address from which the call originates, and the destination address (the called) identifies the remote end point or station with which the originator wishes to communicate.

Zero, one, or more calls can exist on a single address at any given time. A familiar example of multiple calls on a single address is call waiting: During a conversation with one party, a subscriber with call waiting is alerted that another party is trying to call. The subscriber can flash the phone to answer the second caller (which automatically places the first party on hold) and then toggle between the two parties by flashing. In this example, the subscriber has two calls on one address on the line. Because the person at the telephone handset can be talking to only one remote party at a time, only one call is active per line at any point in time. The telephone switch keeps the other calls on hold. With a line able to encompass more than one channel, different configurations can allow multiple active calls on a line at the same time.

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