LANCOM VoIP Router switch calls between different telephone networks, e.g. the ISDN network, various SIP provider networks, and the internal telephone network. These networks generally have different ranges of numbers or even completely different conventions for addressing subscribers. Whereas the traditional land-line network uses numerical characters with country code and area access codes, the world of SIP allows alphanumerical names along with domain information.
The transition between these zones must guarantee the correct translation of "telephone numbers" so that the intended subscriber can be reached. For example, when a call from the land-line network arrives at a public MSN, the requested telephone number has to be translated to the ISDN user's internal number. This act of translation is known as "mapping". Mapping incorporates not only the called number, which represents the destination, but the calling number for the source as well.
Depending on the application at hand, both the called and the calling numbers have to be modified so that a return call can be made to the source number.
Call number translation at the transition to outside lines is primarily implemented by mapping entries in the ISDN and SIP lines and by rules in the call-routing table.