DDIAn extension number allows the extension of a telephone system via public telephone networks to be accessed directly, rather than to be mediated by the attendant need. The extension marks the end of a phone number. English terms in Extension are direct dialing in (DDI) and direct inward dialing (DID).

The assignment of extensions to extensions done by programming the PBX. Usually the extension 0 is used for the inquiry station. If you can forgive multi-digit extensions, is dispensed with each digit extension but to all multi-digit extensions that start with each digit.

In Germany, each system port is assigned to a five digit number block a base number (eg 1234) and at least a single digit number block (0-9) or a maximum. A connection system with a 100 extension block would therefore be as follows: 1234 0 10-99, where the 0 represents the query space. An extension (in the example 15) can be selected with the 123415 (without the 0) can be achieved directly. It is striking that despite the 100 block actually only 90 extensions for extensions are available, as is already 0 reseved for the query space and therefore the extension 01-09 are not available.

dus.net offers all DUStel business rates can activate the DDI function. If this function is used, the dus.net sends the PBX (eg Asterisk) with the called number so that the system can evaluate them and assign the appropriate extension.