The phone numbers transported in an ISDN network have further information in addition to the actual numbers. It thus provides information as to whether a number should be displayed (presentation indicator), how and whether a test takes or has taken place (screening indicator SCRE), to which numbering plan this number belongs (numbering plan identifier NPI), and which codes it contains (type of number TON). In an OpenScape 4000/HiPath 4000 network, the respective node numbers are also transferred together with the numbers ("calling numbers").

Three different numbering plans are supported:

Each node in the network can be set up parallel for all three plans. OpenScape 4000 phone numbers in a numbering plan NPI=ISDN or NPI=PRIVATE consist of the DID number, the various codes, the corresponding prefix and the exit code for the respective numbering plan:

Abbreviations: CC=country code, AC=area code, LC=local destination code, STNO=phone number

The first zero in the example above signals the numbering plan. Traditionally, it means that the subscriber would like an exit to the central office. Since OpenScape 4000/HiPath 4000 networks can also be configured with CO numbers (NPI=ISDN), however, only LCR decides whether a CO exit really takes place or whether a tie line is routed. The exit code (here, 0 for NPI=ISDN) is the only entry in which WABE must be set up (digit analysis result CO or TIE).

The prefix signal the level of a number. In Germany "00" must be dialed to reach international subscribers, for example, in England. The "0" says that a national subscriber, for example, in Dusseldorf, is required. If no prefix is dialed, the subscriber is in the caller's own local network, for example, in Munich. These levels are reflected in the type of number (TON) information. For NPI=ISDN, the values INTERNAT for international, NATIONAL for national, SUBSCR for subscriber-related, and UNKNOWN for DID numbers (e.g. 4711) can be assumed.

The addressing in the respective level takes place using the codes. For NPI=ISDN, the county code (CC) is thus used as the address of a country (e.g. 49 for Germany), the area code as the address of a city (e.g. 89 for Munich), and the local destination code (LC) as the address of the main station (e.g. 7007).

The structure of a phone number in the private numbering plan (NPI=PRIVATE) has the same structure as a number in the ISDN CO numbering plan. The only difference is in the naming of the parameters. The TON can thus assume the values LEVEL2, LEVEL1, and LOCAL and the codes are called Level 2 Code (L2C), Level 1 Code (L1C), and Local Code (LC). Both numbering plans, ISDN and PRIVATE, require that every point in the network is uniquely addressed and can be reached from every other point with precisely this address (e.g. if Germany can be reached with 49 from every point in the worldwide telephony network - only the exit and prefix may differ).

The structure of a phone number in the OpenScape 4000-specific numbering plan (NPI=UNKNOWN) behaves differently. Here, a certain node is addressed with open numbering with cross codes. Here, multiple addressing of a node may result, for example, if two networks are merged. The same node of Point A in the network may have another address - and thus another code - than that of Point B. This may lead to confusion and thus to errors in the execution of network-wide features.

Problems may also arise in the merging of existing networks. In such cases, the solution is to expand the hierarchy by one level so that the node numbers and assigned codes in the network are unique again. If a node in a subnet with the code 99 is connected to the node of another subnet that also has a code of 99, Level 1 is introduced. One node then receives the code 91-99 and the other the code 92-99. The only problem is that the area code 91 or 92 was dialed as well, which means that the numbers change. This has the disadvantage, however, that the numbers 91 and 92 must be unassigned in WABE. If Code 99 is unique in the network, however, it is not necessary to adapt the code when introducing a new level (for example, Node 1-2 would then have a code of 99).

All of these problems are avoided if the ISDN numbering plan is used for these networks. The following table shows the advantages and disadvantages of the numbering plans.