Dial plan

A dial plan establishes the permitted sequences of digits dialed on subscriber or station lines with subscriber premises equipment, such as telephones and private branch exchange (PBX) systems.[1] Dialing plans in the public switch telephone network (PSTN) have traditionally been more commonly referred to as dialing procedures.[2] The dialing plan of a private telephone system or a customer premise equipment, such as an analog telephone adapter (ATA) or an IP phone, is sometimes also called dial plan. The digit sequences (numbers) permissible in a dialing plan 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.

Public switched telephone network (USA)

• Local numbers of seven or ten digits, which may be preceded by a 9, if required to access an outside line.
• Long distance numbers of eleven digits, consisting of a 1, a three-digit area code, and a seven-digit number; preceded by a 9 if required.
• International numbers of any length starting with 011 and preceded by a 9 if required.

Similarly, telephony service operators may provide dialing sequences for special services, such as directory assistance and emergency services.

Private telephone systems

PBX equipment, carrier switching systems, or end-user telephones may specify a variable-length dial plan or a fixed-length dial plan. In private branch exchanges in the U.S. a dialing plan may specify the addresses for the following destinations:

• Internal extension numbers of two, three, or four digits.

Digit maps

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.

Dial Plan Syntax

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:

Dial Plan Syntax Examples

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

See also

• Telephone numbering plan

References

1. John G. van Bosse, Fabrizio U. Devetak, Signaling in Telecommunication Networks, ISBN 978-0-471-66288-4, John Wiley & Sons (2007), p.71
2. AT&T, Notes on Distance Dialing
3. RFC 2705, Media Gateway Control Protocol (MGCP) Version 1.0, Arango et al. (1999)