Extension Numbers
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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.