How does a long-distance call work?

Standard phone lines connect an end user to the local phone company’s central office. The central office computer, called a switch, interprets the numbers dialed and then routes the call to its next destination.


How a long-distance call works.
When the central office detects that “1 + area code” has been dialed and determines the call is to another LATA, it knows the call must be handed off to the long-distance carrier. The central office computer then queries a database to find out which long-distance carrier has been selected for the line. The PIC code is used by the local telephone company’s computers to keep track of a customer’s chosen long-distance carrier. Appendix 10B lists the most commonly used PIC codes.

Once the central office computer determines that the call will terminate outside the LATA, it connects the call to the caller’s chosen long-distance carrier. Although the switching equipment at the local carrier’s central office may be exactly the same as the long-distance carrier’s central office, the long-distance carrier’s central office is almost always called a point of presence. That term refers to the fact that the local carrier has a complete phone network within the LATA, while long-distance carriers only have limited network points within the LATA.

When a caller needs to make a long-distance call, the local exchange carrier’s central office directs the call to the nearest point of presence so the call can then be carried by the long-distance carrier. At the terminating end of the call, the long-distance carrier connects to the local carrier’s central office that serves the person being called.

These multiple interconnections and handoffs between central offices take place within milliseconds. The whole process is seamless to the end user, except for the occasional faint clicking sound that may be the result of antiquated equipment in an older central office.