Editing Telephone numbering plan (section)

=={{anchor|Private numbering plan}}Private numbering plan==
Like a public telecommunications network, a private telephone network in an enterprise or within an organizational campus may implement a ''private'' numbering plan for the installed base of telephones for internal communication. Such networks operate a private switching system or a [[private branch exchange]] (PBX) within the network. The internal numbers assigned are often called ''extension numbers'', as the internal numbering plan extends an official, published main access number for the entire network. A caller from within the network only dials the extension number assigned to another internal destination telephone.

A private numbering plan provides the convenience of mapping station telephone numbers to other commonly used numbering schemes in an enterprise. For example, station numbers may be assigned as the room number of a hotel or hospital. Station numbers may also be strategically mapped to certain keywords composed from the letters on the telephone dial, such as 4357 (''help'') to reach a [[help desk]].

The internal number assignments may be independent of any [[direct inward dialing]] (DID) services provided by external telecommunication vendors. For numbers without DID access, the internal switch relays externally originated calls via an operator, an [[automated attendant]] or an electronic [[interactive voice response]] system. Telephone numbers for users within such systems are often published by suffixing the official telephone number with the extension number, e.g., 1 800 555-0001 x2055.

Some systems may automatically map a large block of DID numbers (differing only in a trailing sequence of digits) to a corresponding block of individual internal stations, allowing each of them to be reached directly from the [[public switched telephone network]].  In some of these cases, a special shorter dial-in number can be used to reach an operator who can be asked for general information, e.g. help looking up or connecting to internal numbers. For example, individual extensions at [[Universität des Saarlandes]] can be dialed directly from outside via their four-digit internal extension +49-681-302-xxxx, whereas the university's official main number is +49-681-302-0<ref>{{cite web |url=http://www.uni-saarland.de/en/footer/dialogue/contact.html |title=Contacting Saarland University |publisher=[[Saarland University]] |archive-url=https://web.archive.org/web/20131120055921/http://www.uni-saarland.de/en/footer/dialogue/contact.html |archive-date=2013-11-20 |url-status=dead }}</ref> (49 is the country code for [[Germany]], 681 is the area code for [[Saarbrücken]], 302 the prefix for the university).

Callers within a private numbering plan often dial a trunk prefix to reach a national or international destination (''outside line'') or to access a [[leased line]] (or ''tie-line'') to another location within the same enterprise. A large manufacturer with factories and offices in multiple cities may use a prefix (such as '8') followed by an internal routing code to indicate a city or location, then an individual four- or five-digit extension number at the destination site. A common trunk prefix for an outside line on North American systems is the digit 9, followed by the outside destination number.

Additional [[dial plan]] customisations, such as single-digit access to a [[hotel]] front desk or [[room service]] from an individual room, are available at the sole discretion of the PBX owner.