A “dial plan” an essential function of every digital telephone network and telephone system. It analyses the number dialled by the call participant and decides which number should actually be dialled or which function should be selected. Thus, with the help of the dial plan it can be determined, e.g., through which provider should a call be placed or to which internal call participant is the call put through. As telephones generally do not have built-in intelligence in terms of establishing connection to a call participant, with the help of the dial plan, the telephone network or the telephone system itself analyses and recognises the dialled number and generates the proper connection request.
Depending on the telephone network or the telephone system, the dial plan can be extremely comprehensive and complex, and it ensures that a whole range of decisions regarding connection establishment can be made. It defines, among other things, to which terminal is the caller transferred, what needs to be done if the call is not answered or the line is busy, and, if necessary, which answering machine has to be informed. In addition, the dial plan basically defines as to what information may be entered by which call participant. It can e.g. prohibit the use of "0" for specific call participants and thus allow only local or internal calls.
It is also possible to block chargeable call numbers with the help of dial plan. In this case, the dial plan can prevent the dialling of numbers beginning with 090, 032 or other numerical sequences and instead forward the call participant to a standard call message.
Dial plan has a specifically defined syntax depending on the type of telephone network or the telephone system model. In certain areas, Asterix has been established as a kind of standard syntax for free telephone system softwares. Asterisk can be run on various computer platforms and supports the Voice over internet protocol (VoIP). The syntax is similar to a script, which is divided into various sections and defines the default specifications for the dial plan using precisely defined strings.