Central office code protection

In the administration of the North American Numbering Plan, central office code protection was a numbering policy intended to ensure that the same local telephone number was not assigned in both of a pair of adjacent communities on opposite sides of an area code boundary. Its primary purpose was initially to preserve seven-digit dialing of local calls across a common area code boundary in border communities.

Central office code protection was once common in communities on provincial or state boundary lines. It has been declining in use as inefficient allocation of numbering resources to the growing number of competitive local exchange carriers has caused depletion of available number prefixes, often requiring ten-digit local calls or overlay plans where multiple area codes serve the same geographic location.

The North American Numbering Plan allocates telephone numbers to central offices in blocks of 10000 consecutive numbers; each block is identified by the area code (three digits) and a central office code (three digits). The remaining four digits identify the line number within the exchange. For allocation to local service providers, these blocks are subdivided and assigned in groups of 1000.

As an example, the entire 506-752 prefix is assigned to Aliant Telecom (NB) and serves Campobello Island, New Brunswick. Campobello is a border community and is a local call to Lubec, Maine in area code 207.

The 207-733 prefix serves Lubec, split between FairPoint Communications and a pair of competitors using number pooling. Both area code 506 and 207 use seven digits for local calls.

A seven-digit local call from Campobello to Lubec is possible, provided that 733-XXXX is not assigned to anything in the 506 area code which is local anywhere near Campobello. This may be done in one of two ways:

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