AT&T MERLIN LEGEND Release 3.1 System Manager's Manual page 373

About Telecommunications
The enhanced CCIS system in use today is called common channeling
signaling (CCS) system and supports advanced features, such as calling-
number identification (Caller ID). The calling party's number is transmitted from
switching office to office. This and other advanced services are available on the
"intelligent" network that has evolved because of the use of computers to
control signaling.
Telephone Numbering Plans
Unlike the verbal communication of the called party's name in manually
switched systems, automated switching systems require that each telephone
station be identified by a unique address that is convenient, readily
understandable, and similar in format to other stations connected to the
network.
In the early days of telecommunications, when a maximum of 10,000 lines could
be serviced by a telephone exchange, a 4-digit alphanumeric "address" was
used to specify the called party, for example, WA51 (Waverly 51).
Subsequently, 3-digit area codes were introduced to specify the area of the
country to be reached, and country codes were introduced for international
dialing. Finally, the individual telephone numbers expanded to the seven-digit
numbers in use today: a 3-digit central office code and a 4-digit station number.
NOTE:
The North American Numbering Plan currently uses area codes that only have a
0 or 1 for the middle digit, for example, 908 or 215. In the very near future, the
numbering plan will be changed so that any number (0 through 9) can be used
for the middle digit. This system has already been designed to take that change
into account.
Various standardized numbers have also been created for special services, for
example, area code 800 for toll-free service, and 911 for emergency service.
A special numbering plan is also used within the MERLIN LEGEND
Communications System to identify individual telephones, adjuncts (for
example, fax machines), trunks, and other features and aspects of the system.
Depending on the number of extensions and the needs of your company, you
can choose from three different numbering plans that allow 2-digit numbers, 3-
digit numbers, or customizable variable-length numbers, respectively.
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System Manager's Guide