How Does Long Distance Work?
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A Customer Chooses a Carrier
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When a customer subscribes to telephone service, he is presented with several options. On a basic consumer telephone line, a long distance carrier--known as an Interexchange Carrier, or IXC--can be assigned for completion of normal long distance calls, no carrier can be assigned so that normal long distance calls will fail but calls dialed with an access code will complete, or long distance can be blocked altogether. If the customer selects a long distance provider, that carrier's four digit code--known as a Primary IXC Code, or PIC--is programmed onto the customer's line in the telephone company's switch. The primary carrier can still be bypassed by dialing a special access code before a long distance call, but all normal calls will be routed through that carrier.
Calls Are Routed to the IXC
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When a customer makes a long distance call, the telephone company switch determines the destination of the call based off of the dialed area code and prefix; depending on the switch model and software version, the presence of a "1" before the area code may also help identify a long distance call. When the switch identifies the call as long distance, it hands off the call to the selected long distance carrier; certain identifying information--such as the originating telephone number, the number dialed, and the time the call originated--is handed off as well. Using the dialed number as a reference, the long distance carrier sets up a connection between the calling party and the called party's local telephone company switches; depending on the carrier and the resources available, this connection may be made using either physical circuits, microwave radio transmission, or packet-based (Voice Over Internet Protocol, or VOIP) technology.
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The Local Telephone Company Signals the Destination Company
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While the long distance company is setting up the actual channel for the conversation to take place, the local telephone company performs additional work behind the scenes. Like the long distance carrier, the caller's local telephone company uses the area code and prefix (known to those in the industry as the NPA/NXX) to identify the destination of the call. Using the information, the local company signals the remote telephone switch that will ultimately complete the call, alerting it to an incoming call and transferring several pieces of information. Depending on the capabilities of the destination switch, signaling information is used to drive services like Caller ID, *69 (return call) and Call Trace. In outage conditions or in cases of poor connection quality, the signaling information can also be used to help both telephone companies identify and correct trouble spots.
The Remote Telephone Company Completes the Call
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When the long distance carrier connects to the called party's local telephone company, the call is again handed off to the switch that provides service to the called party. Regardless of the technology used by the long distance carrier to carry the call to the destination, most local telephone companies make the connection between the long distance carrier and the called party using either a physical circuit or VOIP, depending on the called party's type of service. When the called party answers, the circuit is completed and the two parties are connected to one another. The connection remains in place until either party hangs up, at which time the long distance carrier and both local telephone companies remove the temporary circuit and wait for another call.
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