Recommendation E.164 of the International Telecommunications Union (ITU) also defines the National Numbering plan as the document specifying the format and structure of the numbers used within this plan. It stipulates the distribution of the digit groups, so as to highlight the specific elements by which the functions of identification, routing and tariff-charging could be implemented (for example, within E.164: identification of the country, of the national called points, and of the subscribers).
The national numbering plan does not comprise the prefixes, suffixes and additional information required for initiating a call. The numbering plan comprises E.164 numbers, prefixes (national, international and carrier selection indicatives) and "non-E.164" numbers (e.g.: "short" local numbers).
For the provision of additional information, one may use the codes * and #. These are used in the control procedures initiated by a subscriber (e.g.: call re-routing). Although these codes do not identify the network terminal points and have only a local significance, they may be included in the national numbering plan. For the terminals that do not have the * and # keys, these codes may be replaced by digit sequences.
The national numbering plan (NNP) transposes, according to the same document, on national level, the E.164 numbering plan.
NNP types: closed / open. The NNP deals with two categories of numbers, considering the digit sequence dialled for initiating a call: closed NNP – dialling the whole national significant number, disregarding the calling or the called telephone set (including local calls). Closed numbering is used, for example, in the mobile telephone networks. Open numbering and, respectively, the open NNP do not correspond to the above specifications.
Geographic and non-geographic numbers. A geographic number is a number established according to the National Numbering Plan, within which one or several digits contribute to identifying the geographic location of the network terminal point. A non-geographic number is a number established according to the National Numbering Plan, which is not a geographic number; non-geographic numbers are, among others, the numbers allotted to mobile telephony services, green numbers and premium rate numbers.
Dialling Plan. Taking into account the open character of the Romanian NNP, the method of dialling the digit sequence for initiating calls is presented in the so-called dialling plan. Generally, this sequence has the following structure: ICSP IP CC NDC SN,