3. The extension as the key element

In order to understand the system and its configuration, it is essential to understand that the extension is the basis of the telephone system. A number is not assigned to a person or a telephone but to the extension. In turn, this extension can be assigned to a destination. Possible destinations are:

  • Telephones (or a fax machine)
  • An eFax
  • Services (such as queues)

An extension is always the internal number, i.e. this number is used to reach parties and also services within the company without dialling the outside line prefix (leading “0”). This internal number is always completely independent of the direct dial number dialled from outside. That is, extensions cannot be valid at all from outside or in theory, with a different number sequence. In the case of telephones, it is usually typical for the internal telephone number (extension) to also correspond to the external telephone number (direct dial number).

Quick relocation…
A user’s personal settings are assigned to the respective extension (not to the telephone!). This enables being able to “relocate” the properties of an extension to another telephone without having to reprogram the settings. Consequently, the extension is virtually the “soul” of a telephone.
This function is also referred to as “hot-desking”. For example, you work from several locations and always use the same extension.


From Monday to Wednesday you work at site A and Thursday to Friday you work at site B. You can easily use the same extension by selecting the respective end device for the location on the service site, when configuring your extension.