Connections via EURO-ISDN exchange lines transport various info-numbers in as additional connection information, which include the "calling line information" and the "connected line information" (respectively shortened to "calling number" and "called number"). These info-numbers not only appear in the telephone displays of the connection parties; many system features use these info-numbers as destination numbers at later stages in a call setup, e.g. for recall, callback, etc. For this reason, an Info-Number Signalling system exists (INS), whose function is to ensure that any node-specific modification of these numbers for transport is carried out in such a way that the info number string (including the necessary route codes etc.) can be reconstructed by any system or node at any stage in the connection setup path. Networks transport the info-numbers in explicit format throughout, wherever possible. Explicit info-numbers do not contain the full dialled number string, but only the root number and additional information elements indicating the breakout codes, prefix codes, area codes, the numbering plan identifier (NPI=ISDN for EURO-ISDN) and Type of Number (TON, i.e. INTERNAT, NATIONAL or LOCAL). Since call number information of this type is not suitable for display or further processing within a system or node, each node converts incoming explicit calling number information into an implicit format for internal purposes. The implicit calling number information will contain all the appropriate code digits (breakout and prefix codes) and have the type of number attribute TON = UNKNOWN. In the outgoing direction, the implicit format is converted back into an explicit format. The conversion is always carried out with the station number modification function, i.e. depends on AMO settings specific to each node. The following example shows an ISDN exchange and a destination node, with the intervening network.

Two parallel methods exist for modifying the calling number information (calling line number and connected line number): the new format-dependent station number modification (AMOs KNFOR, KNPRE and TDCSU) and the 'old' source/destination-dependent method as administered with the KNMAT AMO. Hicom 300 E V1.0 nodes with the ISDN numbering plan (NPI=ISDN) should always use the format-dependent method. To this end, the ISDN exchange must be assigned a virtual node number in order to be able to configure the outgoing trunk groups for routing (DNNO parameter of RICHT AMO) and incoming trunk groups for identification (NNO parameter of TDCSU AMO). The own node number must always be entered in the central PABX data administered with the ZAND AMO (parameter NNO). The virtual node number must have the same hierarchical level structure (i.e. single-level, 2-level or 3-level) as the NNO specified in the ZAND AMO.