Intelecommunication, a dial plan (ordialing plan) establishes the permitted sequences of digits dialed by telephone subscriber and the manner in which a telephone switch interprets these digits within the definitions of the prevailing telephone numbering plan.[1] Dial plans in the public switched telephone network referred to as dialing procedures.[2]
The collection of permissible digit patterns, so called digit-maps, for a private telephone system or for customer premise equipment, such as an analog telephone adapter (ATA) or an IP phone, is sometimes also called dial plan. A pattern may be as short as a single digit, e.g. for reaching an operator, or as long as a complete international telephone number, including trunk prefixes and international prefixes.
Similarly, telephony service operators may provide dialing sequences for special services, such as directory assistance and emergency services.
PBX equipment, carrier switching systems, and end-user telephones may use variable-length or fixed-length dial plans. In private branch exchanges in the U.S. a dialing plan may specify the addresses of internal extensions, typically as numbers of two, three, or four digits.
Dialing from a PBX telephone is usually by default to internal numbers, i.e. for calls within the PBX, so that reaching PSTN numbers often requires dialing a special prefix, e.g. a 9, before the PSTN number.
Analog telephone adapters, IP phones, and many other VoIP media gateways have configuration options that establish the digit sequences that can be dialed with the equipment. The dial plan of these devices is established by a digit map. The following syntax may be used for such dial plan, as adapted from RFC 2705,[3] the specification for the Media Gateway Control Protocol.
To specify a |
Enter the following |
Result |
|---|---|---|
| Digit |
0 1 2 3 4 5 6 7 8 9 *
|
Identifies a specific digit (do not use #) |
| Range |
[digit-digit]
|
Identifies any digit dialed that is included in the range |
| Range |
[digit-digit, digit]
|
Specifies a range as a comma separated list |
| Wild card |
x
|
x matches any single digit that is dialed |
| Wild card |
.
|
. matches an arbitrary number of digits |
| Timer |
T
|
Indicates that an additional time out period of 4 seconds should take place before automatic dialing starts |
Some dial plan examples using the above syntax look as follows:
For calls to |
Users dial |
Dial plan |
|---|---|---|
| Internal Extension |
a two digit number
|
xx |
| Local Number |
9 (if required for an outside line) and then a seven digit number
|
9xxxxxxxT |
| Emergency |
911
|
911 |
| Local Operators |
9 (if required for an outside line) then 0
|
90T |
| Long Distance |
9 (if required), 1, area code and local number
|
91xxxxxxxxxx |
| International |
9 (if required), 011, any number of additional digits
|
9011x.T |
