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Number portability is the practice of allowing customers to transfer their telephone numbers from one telephone operator (PSTN, mobile, IP-based or other) to another.
Number portability became popular with the advent of mobile telephones, since in most countries different mobile operators are provided with different area codes and, without portability, changing one's operator would require changing one's number. Some operators, especially incumbent operators with large existing subscriber bases, have argued against portability on the grounds that providing this service incurs considerable overhead, while others argue that it prevents lock-in and allows them to compete fairly on price and service. Due to this conflict of interest, number portability is usually mandated for all operators by telecommunications regulatory authorities. The ability to keep a number while switching providers is thought to be attractive to consumers.
National status
The world's first country to introduce number portability for mobile telephones was Singapore in 1997. In the United States, the FCC has mandated Wireless Local Number Portability starting November 24, 2003 (in metropolitan areas), and allowed operators to charge an additional monthly Long-Term Telephone Number Portability End-Use Charge as compensation. On November 10, 2003, the FCC additionally ruled that number portability applies to landline numbers moving to mobile telephones as well.
In the European Union, all telephone providers are required to provide number portability under the Universal Services Directive (2002/22/EU).
In Australia, mobile telephone numbers have been portable between carriers since 2001. Previously, prefixes 04x1, 04x2 and 04x3 referred to Optus customers, 04x4, 04x5 and 04x6 referred to Vodafone numbers, and 04x7, 04x8 and 04x9 referred to Telstra. Portability has been a great asset in allowing freedom of choice of service provider. However many telecommunications companies had special or discount rates for calls between two customers of the same service provider, which although the special rates still existed, it was made difficult for customers to determine which provider the person they were calling was with. Land line phone numbers are tied to a particular physical telephone exchange - as of 2004 there are no plans or desire to change this.
In Canada, wireline/competitive local exchange carriers must provide portability but most mobile carriers do not (the one exception as of 2004 being Microcell Fido)
Technical issues Complexity for number portability can come from many sources. Historically numbers were assigned to various operators in blocks. The operators, who were often also service providers, then provided these numbers to the subscribers of telephone services. Numbers were also recycled in blocks. With number portability, it is envisioned that the size of these blocks may grow smaller or even single numbers. Once this occurs the granularity of such operations will represent a greater workload for the telecommunications provider. With phone numbers assigned to various operators in blocks, the system worked quite well in a fixed line environment since everyone was attached to the same infrastructure. The situation becomes somewhat more complex in a wireless environment such as that created by cellular communications.
In number portability the “donor network” provides the number and the “recipient network” accepts the number. The operation of donating a number requires that a number is “snapped out” from a network and “snapped into” the receiving network. If the subscriber ceases to need the number then it is normal that the original donor receives the number back and “snaps back” the number to its network. The situation is slightly more complex if the user leaves the first operator for a second and then subsequently elects to use a third operator. In this case the second operator will return the number to the first and then it is assigned to the third.
In cellular communications the concept of a location registry exists to tie a “mobile station” (such as a cellular phone) to the number. If a number is dialed it is necessary to be able to determine where in the network the mobile station exists. Some mechanism for such forwarding must exist. (For an example of such a system, see the article on the GSM network.)
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