AT&T MERLIN LEGEND System Reference Manual page 92

Features
How ARS Works
Other digits or special characters may be required so the system can route a
call on a particular trunk pool. For example, some companies use an alternate
toll company that requires dialing of the telephone number with pauses and
access codes. After the user dials the telephone number, the system
automatically adds the required pauses and access code to the front of the
telephone number, based upon the particular table used. A maximum of 20
digits can be added to numbers dialed by users.
From 0 to 11 dialed digits can also be programmed to be deleted or absorbed
by the system before the dialed number is sent to the central office. For
example, if the central office does not require dialing a 1 before an area code,
the system can be programmed to absorb that first digit. The system absorbs
digits starting with the first digits dialed by the user.
To make a call in a system that has ARS, a user selects a System Access button
and dials the ARS access code (usually a 9) followed by the telephone number.
The call is routed over the trunk pool that is the most cost effective. The system
determines the best route for the call by checking the telephone number dialed
against the numbers (digits) in the ARS tables. ARS eliminates the tables until it
finds one containing a match. A match can be made by comparing one, three,
or six digits.
The system checks to ensure that the user has the appropriate permission for
making each call (including a check for Allowed and Disallowed List
assignment). First the dialed number is checked to see if it is an emergency
number-for example, fire or police. If so, the call is routed over the main pool,
regardless of any restrictions. If it is not an emergency call, the permissions are
checked for any call restrictions (outside or toll).
If a user dials "0," ARS eliminates all tables except the Dial Zero table and routes
the call by using the table's programmed routes. If a user dials the equal access
code (10xxx), the call is routed through the main pool.
If a user dials an 11-digit number that begins with 1 (for example, 1-201-555-
1234), ARS first recognizes that it is a toll call (a 1 followed by 10 digits) and
eliminates all exchange tables.
ARS then checks the area code tables and 6-Digit tables for a match. If it finds a
match on a 6-Digit table, it routes the call accordingly. If it does not find a match
on a 6-Digit table, it checks the Area Code tables for a match and routes the
call. If a match is not found, the call is routed according to the default toll table.
Once ARS selects the appropriate routing table, it checks the time of day to
select the appropriate subpattern and tries to route the call via the first route
programmed for the subpattern. ARS then checks the FRL assigned to the first
route and to the telephone from which the call is being made.
Automatic Route Selection 2-43