In 1996, the U.S. Congress, mandated new regulation that provides for a local number portability (LNP), which simply means when a consumer’s phone service is moved from one service provider to another service provider, the telephone number (TN) does not change. The process is quickly defined as porting a number.
Location Routing Number (LRN) is a ten (10) digit number following the North American Numbering Plan (NANP), stored in a database and each ported TN is assigned a LRN that specifies the updated and correct service provider.
LRN is called a “single-number solution” as only one number – the number dialed by the calling party – is used to identify the customer in the service provider’s switch.
Please note well that the frequency of ported numbers and assigned LRNs has millions of changes in the U.S. on a daily bases. The LRN database is updated near real time in most cases and certainly within hours of the change in service provider.
You can see the original 1996 U.S. Congress Document here.