PBX often uses an internal numbering system for every phone, which are all internally connected. The numbers assigned to each phones are generally unique and three or four digit in length. These phones allotted with numbers are generally what we refer as “Extensions“. These extensions are generally followed by company’s business numbers allotted by the telephone service provider.
Lets take an example to understand it in more detail. The ABC company has 20 employees. Each employee have a phone on his/her desk. These phones are connected with the PBX server of company and allotted with special extension numbers like 301,302,303….upto 320. Now employee having extension 301 can directly call to any extension just by dialing the extension number. But if any body outside of company (or if we say any body not connected to PBX) want to call specific employee, he must need to dial employee’s extension number followed by company’s business number, for example 6643-301, 6643-309, 6643-320 etc.