PBXs are distinguished from smaller “key systems” by the fact that external lines are not normally indicated or selectable at an individual extension. From a user’s point of view, calls on a key system are made by selecting a specific outgoing line and dialing the external number. A PBX, in contrast, has a dial plan. Users dial an escape code (usually a single digit; often the same as the first digit of the local emergency telephone number) that connects them to an outside line (DDCO or Direct Dial Central Office in Bell System jargon), followed by the external number. Some modern number analysis systems allow users to dial internal and external numbers without escape codes by use of a dialplan which specifies how calls to numbers beginning with certain prefixes should be routed. It’s a good choice when you choose small business VoIP as a start for your small business.

A mobile PBX is a hosted PBX service that extends fixed-line PBX functionality to mobile devices such as cellular handsets, smartphones and PDA phones by provisioning them as extensions. Mobile PBX services also can include fixed-line phones. Mobile PBX systems are different from other hosted PBX systems that simply forward data or calls to mobile phones by allowing the mobile phone itself, through the use of buttons, keys and other input devices, to control PBX phone functions and to manage communications without having to call into the system first.

A mobile PBX may exploit the functionality available in smartphones to run custom applications to implement the PBX specific functionality. In addition, a mobile PBX may create extension identifiers for each handset that allow to dial other cell phones in the PBX via their extension shortcut, instead of a PSTN number.