Extension Numbers

An extension number is composed of one or more digits in the range 0 to 9 and must be unique within a site, although the same extension number can exist in multiple sites (that is, overlapping extension numbers). The length is determined on a site-by-site basis.

Extension number ranges chosen should not overlap with the intersite prefix or with PSTN access prefixes. To prevent overlap, do not use extension number ranges starting with a PSTN access prefix such as 9, or the chosen intersite prefix, commonly 8. Overlap between extension numbers and the emergency number at any location must be avoided. For example, if the emergency number is 112, then extension 112 or extension number ranges 112X (where X is one or more digits) are not permitted.

For a Flat Dial Plan (G2), the extensions and the internal DNs are the same. Extensions under Flat Dial Plan (G2) cannot overlap and have to be unique across all locations for a customer.

For a Type 4 Flat Dial Plan, the extensions and the internal DNs are the same. Extensions under Flat Dial Plan cannot overlap and have to be unique across all locations for a customer.