A Telephone Number is Assigned and Programmed
Before a telephone company can assign telephone numbers, it must first purchase a block of numbers from the North American Numbering Plan Administration's registrar, a company known as Telcordia. Telcordia generally assigns an entire block of 10,000 telephone numbers (one area code and prefix combination, known as an NPA-NXX), though some smaller companies purchase only 1,000 numbers at a time. The registrar then declares the purchase in a monthly database release that telephone companies program into their equipment, usually into the service control points (or SCPs) in their signalling system.
A Caller Places a Call
When a caller places a long-distance or local toll call from one of the local telephone company's lines, the telephone network queries the service control point to determine which telephone company will receive and complete the call. If the NPA-NXX has been properly entered into the service control point, the SCP will return a routing instruction that directs the call to the proper telephone company for completion. Because SCPs can process thousands of queries every second, this process is transparent to the caller.
About Number Portability
While the established process for programming and querying SCPs has been effective for routing based on NPA-NXX, the ability to port numbers to other telephone carriers complicated the querying process. Since individual lines may be moved to other carriers, it is no longer always true that a call to an NPA-NXX will complete to the telephone company that originally purchased the block of numbers. For this reason, Telcordia now coordinates routing based on individual line "exceptions." While full NPA-NXX combinations are still loaded into the SCP for routing queries, individual numbers are now also loaded into the SCP. When the telephone company's signaling equipment queries the SCP for routing instructions, it first checks for the full 10-digit number. If there is no special routing instruction for the specific number, the call is routed to the default carrier for that NPA-NXX. If the number does exist as an exception in the SCP, the call is instead routed to the carrier to which that number now belongs.
Tags: telephone company, service control, block numbers, call NPA-NXX, control point