Obtaining a Block of Numbers
All telephone numbers in the United States and Canada are administered by Bellcore/Telcordia and the North American Numbering Plan Administration (NANPA). These organizations assign sets of telephone numbers in blocks of 10,000 numbers at a time (and entire prefix), though some smaller companies may qualify for "thousand block pooling" in which only 1,000 numbers are allocated for a smaller fee. The area code, or NPA, and the prefix, or NXX, are traditionally tied to a geographical location, though this practice may be changed by local number portability and the emergence of Voice over IP (VoIP) technology.
Assigning a Random or Nearly Random Number
When a customer signs up for service, mobile phone companies assign a number from a pool of available numbers. Generally, these numbers are assigned at random, though sometimes a helpful employee may offer a choice of numbers to the customer. The available numbers are drawn from the blocks of numbers assigned through Bellcore/Telcordia and NANPA and consist of numbers not currently in use or which have not been used within the preceding 90 days.
Porting Your Old Number
For customers who already have an existing telephone number and do not wish to release that number, the number may be reassigned to the new mobile carrier through a process known as "porting." If a customer chooses to port her number, employees of the new cell phone company will complete paper work to have the number released to their company.