AT&T MERLIN LEGEND System Reference Manual page 413

Features
Mode Differences
Considerations
2-364 System Numbering
As shown in Figure 2-21, the system reassigns extension numbers in the 7000
range for all stations. This makes numbers beginning with 1 through 6 available
for use in reassigning extension numbers. These new extensions can be from
one to four digits long.
The extension numbers shown for MFMs/7500Bs (7300-7443) are reserved for
digital station jacks. These numbers are automatically assigned by the system to
adjuncts (such as a data terminal, answering machine, or fax) connected to an
MLX telephone using an MFM or ISDN 7500B Data Module. The actual
extension number assigned to the adjunct is the extension number assigned to
the MLX telephone increased by 200. For example, if the extension for an MLX
telephone is 7125, the extension for the adjunct on that telephone is 7325. In this
example, a call can be made to the telephone by dialing 7125 or to the adjunct
by dialing 7325.
In the Hybrid/PBX mode, an extension is assigned to the Listed Directory
Number (the published main number) for the QCC queue. The factory setting is
800.
Extensions do not need to be renumbered in the following cases:
The preset two-digit extensions are acceptable.
There are fewer than 50 stations in your system.
No special extension numbers are needed.
Any extension number except 0 (system operator) can be renumbered.
Line/trunk numbers (801-880) are not extensions and cannot be
renumbered.
Extension numbers can contain the digits 0 through 9 in any combination,
except that no code can begin with 0. (0 is a fixed dial code representing the
primary system operator or QCC queue.)
Extension numbers can contain one to four digits and must be unique. If an
extension number is assigned with one or two digits, those digits cannot be
used as the leading digits for a longer extension number. For example, if
extension numbers 1, 2, 30, and 40 are assigned to telephones, those
numbers cannot be used as the first number in longer extension numbers,
such as 10, 200, 302, or 4052.
Any time an extension number is reassigned, the original extension number
is available for use. However, the corresponding station must be assigned a
new extension number before the original extension number can be used for
anything else.
A limited number of extensions can be changed through programming
without altering the overall two-digit numbering scheme.