A PBX (Private Branch Exchange) is a telephone exchange system installed within a business or organization. It provides functions including external/internal line connection, extension-to-extension calls, transfers, hold, and conference calls, serving as the core of corporate telephone infrastructure. Without a PBX, each employee would need their own external line, so the number of contracted lines would scale directly with headcount.
The basic operation of a PBX is receiving incoming external calls and connecting them to the appropriate extension, and connecting outgoing extension calls to external lines. Combined with dial-in, it enables direct department numbers. Combined with IVR, automated voice routing becomes possible. PBX systems equipped with ACD are essential for call centers, automatically distributing calls to operators based on wait time and skills.
PBX systems fall into three main categories. First, traditional on-premises PBX: dedicated hardware installed in the office, connected to analog or ISDN lines. Buying the hardware and having it installed makes the up-front cost substantial, but because the voice path never crosses the internet, it is unaffected by internet line outages. Second, IP-PBX: uses VoIP technology, connecting IP phones via LAN cables. Existing LAN infrastructure can be reused, so there is less need to lay dedicated telephone cabling, and calls between offices can be carried as extensions over an IP network. Third, cloud PBX: no on-site hardware, used as a cloud service. Nothing has to be installed on site, and it lends itself to using smartphones as extension handsets even in remote work environments.
Selection criteria vary by company size, number of locations, and work style. Offices of roughly ten employees or fewer find cloud PBX convenient, while sites of more than a hundred employees tend to prioritize the stability of on-premises or IP-PBX systems. Companies with multiple locations usually consider IP-PBX or cloud PBX, which let calls between offices be handled as extensions. Review selection points in the business phone number guide.
Everyday PBX features include free extension-to-extension calls, transfer and hold, "call distribution" that routes calls to the main number across multiple phones simultaneously or in sequence, and "call pickup" that lets a nearby colleague answer a call ringing at an unattended desk. With IP-PBX and cloud PBX, on the other hand, calls depend on the office LAN or internet line and on mains power, so decide before deployment where calls to the main number will be answered during a power or line outage.