Nortel VoIP Gateway, Configuration Manual, Page: 148 / 220

148 Dialing Plans
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In North America, the national number is further broken down into a national destination code,
called the area code, and a subscriber, or local number. In some large metropolitan areas, you must
dial both the area code and local number on each local call.
Private numbering plans are not regulated and therefore can take many different forms. In general,
a private numbering plan is setup to minimize the number of digits needed to dial local calls in the
private system. For example, you may have a 3 or 4 digit numbering plan for local calls within
your company. This is equivalent to the local, subscriber number in the national numbering plan.
Private numbers can also use the equivalent of a public area code. Nortel Networks ESN
(Electronic Switched Network) is an example of this. ESN numbers, like area codes, generally
refer to geographic locations or separate offices.
Dialing Plans
There is a difference between a Numbering Plan and a Dialing Plan. The numbering plan defines
the unique numbers that represent each voice call endpoint. The dialing plan defines rules
(different in each country) for how those numbers are dialed and are used to instruct the telephone
system (public or private) on how to route the call.
For example, in North America, you do not need to dial the North American country code when
dialing another North American subscriber. What you dial is the National Number. In order to tell
your phone company that you wish to dial long distance, you must first dial a long distance access
number, which is part of the public dialing plan. Traditionally, this is the digit "1". You are not
dialing the country code of 1, you are dialing the long distance access code used in your telephone
company's dialing plan. Confusing, isn't it! However, in this time of open competition for long
distance, the long distance access number may be other codes.
In enterprise voice networks, dialing plans are used to route traffic locally, to remote enterprise
locations and into the PSTN. For example, you might use the digit "9" to indicate that the call is for
the public network. You might use the digit "6" to indicate that the call is destined for a location at
another office of your company. Finally, to reach local telephones within the business, you would
simply dial the local number.
For dialing plans to work, they must be compatible with the numbering plan. For example, if you
use the digit "9" to indicate that you want to dial a PSTN call, you cannot use the "9" as the first
digit in any of you numbering plan. Same goes for the digit "6". You'll notice that this rule is also
present in the public network. There are no area codes or local numbers that start with the digits 1
or 0 because these digits are reserved for special meaning within the dialing plan.