The 877 prefix code is used for toll-free telephone numbers in the North American Numbering Plan, alongside prefixes such as 800, 844, 855, 866 and 888. Toll-free numbers are commonly registered by businesses so that customers can call without being charged for the connection. Calling a toll-free number from a mobile phone still incurs airtime charges with the cellular service provider.
The first toll-free prefix, 800, was instituted into the NANP in 1966. This replaced the preceding Zenith number system, used in the 1950s. It partially automated a reverse collect-call system, allowing operators to connect the call and bill the called party instead of the calling one. The 800 prefix was the sole toll-free number in service for almost 30 years before the 7.8 million unique telephone numbers for it were exhausted.
Toll-free telephone numbers with prefixes such as 877 are not location-specific, as with area codes. Instead, these numbers are referred to as non-geographic numbers. They are assigned to requesting parties on a first come, first served basis by a Federal Communications Commision-designated Responsible Organization. These entities, which are primarily but not entirely telephone companies, have access to the SMS/800 database. As of 2015, the database is maintained by Somos, per the FCC SMS/800 Functions Tariff.