Glossary of terms - Prefix

The possibility to reach most of the subscribers of a branch exchange from an external telephone network with a common telephone number.  This is usually done by handing over a few last digits to the branch exchange, which processes them and rings a specific calling party without the necessity of being put through by an operator. For the implementation of the prefix, a special type of connection is needed, either the so-called analog three-wire prefix, the PCM or ISDN connections. A prefix cannot be fully implemented on standard country lines, only in the form of a  DISA extension.