Avaya Communication Manager Administrator's Manual page 872

Screen Reference
Valid entries
fac
pext
872 Administrator Guide for Avaya Communication Manager
Usage
Feature access code only — A FAC can be any number from 1 to 9
and contain up to 4 digits. You can use * or #, but only as a first digit.
Avaya recommends that a FAC have the longest total length for a
given dialed string when using mixed numbering. Otherwise,
problems may occur when, for example, 3-digit FACs and 4-digit
extensions begin with the same first digit and the FAC is an
abbreviated dialing list access code.
However, if the entry in the dial plan that defines the FAC is used to
define the AAR or ARS access code, then it must have the longest
total length in the dial plan.
—
Prefixed extension
to 9 (* and # not allowed) and an extension number of up to 5 digits in
length. The maximum length of a prefix and extension combination is
6 digits. You cannot administer a dial access code with the same first
digit as a prefixed extension.
The purpose of the prefix is to identify the call type as an extension.
After digit collection, the prefix digit is removed from the string of
dialed digits. The remaining digits (extension number) are then
processed. A prefixed extension allows the use of extensions
numbers with any dialed string (the extension length must be
specified on the table). The "prefixed extension" cannot have the
same dialed string as the ARS or AAR facility access code (FAC).
Is made up of a prefix (first digit) that can be a 0
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