PBX phone systems operate essentially as a link between a private firm or business and the public switched phone network. Since they incorporate phones, fax, modem, and other relevant machines,  the term "extension" is utilized for referring to any end point on the branch. The PBX phone systems handle calls between the above mentioned extensions and also connections to the PSTN by means of trunk lines. The former is different from key systems in a way that the user of the key systems needs to manually choose the outgoing lines. On the other hand, PBX phone systems pick the outgoing line on their own whereas hybrid systems combine features of both the PBX phone systems and the key systems. Even a small system can have all relevant features like voicemail, ring groups, music on hold and auto attendants. These features make PBX phone systems a complete one.

A contemporary system comes with several sophisticated call handling features, which give you complete control over your calls. Hence, you can check the list of features before you select a particular system. The idea is to maintain your work flow at the office, by keeping the communication linkages robust and flexible. Any telephone and that too from wherever, can easily be made a remote extension of your phone systems. Cell phones, landlines at home and other office branches can be reached by dialing a particular number. Also, a seamless flow of call transfers and screenings can be maintained with PBX phone systems.