Architecture

PBX (Private Branch Exchange)

A PBX is a private telephone switch that routes calls between extensions inside an organization and connects to outside lines (PSTN trunks) for external calls. In the legacy AT&T switching hierarchy this is called a Class 5 switch (end-office, subscriber-facing, feature-rich) — as opposed to a Class 4 switch (wholesale trunking carrier, no end-user features). See Class 4 vs Class 5 for the full distinction. Originally analog hardware (1900s through 1990s), modern PBXs are software running on commodity servers (IP-PBX) or in the cloud (hosted PBX).

Three eras

1. TDM PBX (1900s-1990s): hardware switch with analog or T1/E1 trunks. Vendors: Avaya, Nortel, Mitel, NEC.
2. IP-PBX (2000s+): software running on Linux/Windows. SIP trunks instead of T1. Vendors: Asterisk/FreePBX, 3CX, FreeSWITCH, Cisco CUCM.
3. Hosted/Cloud PBX (2010s+): the PBX itself is multi-tenant cloud software. RingCentral, Zoom Phone, Microsoft Teams Phone, Google Voice for Business.

Core PBX features

• Internal extensions, transfer, hold, conference
• Auto-attendant / IVR
• Call queues, ring groups, hunt groups
• Voicemail and voicemail-to-email
• Call recording
• Outbound caller-ID rewriting
• Inbound DID routing

PBX vs VoIP

VoIP is just the transport (calls over IP rather than TDM). A PBX is the application layer that handles call control. You can have VoIP without a PBX (a softphone calling a SIP trunk directly) or a non-VoIP PBX (a 1990s analog PBX). Modern systems are usually both.

Related terms