PBX stands for – Private Branch Exchange

PBX  ( Private Branch Exchange ) is a private telephone network used within a company. The users of the PBX phone system share a certain number(s) of outside lines for making external phone calls and each one can have their internal extension number.

A PBX connects the internal telephones within a business and also connects them to the public switched telephone network (PSTN).
Most medium-sized and larger companies use a PBX because it’s much less expensive than connecting an external telephone line to every telephone in the organization. In addition, it’s easier to call someone within a PBX because the number you need to dial is typically just 1 to 4 digits

One of the latest tendencies in PBX phone system development is the VoIP PBX, also known as IP PBX, which uses the Internet Protocol to transmit calls.

Nowadays, there are four different PBX phone system options:

PBX
Hosted/Virtual PBX
IP PBX
Hosted/Virtual IP PBX

IP PBX is a software-based PBX phone system solution which helps accomplish certain tasks and delivers services that can be difficult and costly to implement when using a traditional proprietary PABX.

One of the PBX advantage for companies is using PBX technologies to connect each internal number to an external number ( can be local or international ), this is the technology use in many call center, you dial a local number and after you enter the extension you will transfer to another number ( may be in different country or even a cellphone ), using PBX technologies in call centers operator can easily transfer your call to any internal or external number just with a click.