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Glossary · Voice technology

What is PBX (private branch exchange)?

  • Also called: private branch exchange, phone system, cloud pbx

Reviewed by Sujan ThapaliyaLast updated

Definition

A PBX is the private switch that connects a business's internal phones to each other and to the public network. Originally a cabinet in a cupboard, it is now usually software — hosted by a provider, with handsets and softphones registering to it over the internet.
The three STIR/SHAKEN attestation levels, which underpin how pbx (private branch exchange) behaves in practice: A asserts both that the carrier knows the customer and that the customer may use the calling number, B asserts the customer only, and C asserts neither. A shared number pool can reach B at best, which is why number ownership keeps recurring in these definitions.
Attestation is the backdrop to most caller-ID and deliverability terms, pbx (private branch exchange) included.

In practice

How pbx (private branch exchange) actually works

Everything a business expects from a phone system is a PBX function: extensions, transfers, hold, hunt groups, voicemail, auto attendant. The move to hosted changed where it runs, not what it does.

The practical difference is capacity. An on-premises PBX had a fixed number of lines and required an engineer to add more; a hosted one adds a number or a seat as a configuration change.

A hosted PBX and a SIP trunk into an existing PBX solve different problems: the first replaces the system, the second replaces the lines behind it.

The hosted model also changes who is accountable when something breaks. An on-premises PBX makes the fault yours and the fix yours; a hosted one makes diagnosis a conversation with a provider. Neither is strictly better, but the second requires a provider who will actually answer, which is worth testing during evaluation.

Worked example

Adding a line to an on-premises PBX historically meant hardware and an engineer visit. On a hosted one it is a configuration change, which is why capacity stopped being a procurement question.

PBX (private branch exchange): common questions

What is the difference between a PBX and a phone system?
None, in ordinary use. PBX is the technical name for the switch a business phone system runs on, whether it is a box on site or software in a data centre.
Is a cloud PBX better than an on-premises one?
For most businesses, yes — no hardware to maintain, capacity that changes in software, and remote workers who are ordinary extensions. On-premises still suits sites with specific regulatory or connectivity constraints.
Can I keep my PBX and just change the lines?
Yes. That is what a SIP trunk does: it replaces the physical lines behind an existing PBX while leaving the system and handsets in place.

Sources

  1. Combating Spoofed Robocalls with Caller ID Authentication — Federal Communications CommissionThe STIR/SHAKEN framework, the attestation levels carriers sign calls with, and the mandate requiring providers to authenticate caller ID.
  2. 47 U.S.C. § 227 — Restrictions on the use of telephone equipment — Cornell Legal Information InstituteThe Telephone Consumer Protection Act itself — the consent requirements, calling-hours limits, and private right of action.
  3. ITU-T Recommendation E.164 — The international public telecommunication numbering plan — International Telecommunication UnionThe international number format, the 15-digit maximum, and how country codes and national numbers compose.

See pbx (private branch exchange) in the product

SIP trunking carries your phone calls over the internet to and from your existing PBX, replacing physical ISDN or PRI lines. You keep your phone system and your numbers; you drop the copper, the per-channel rental, and most of the long-distance bill.

Explore sip trunkingGet early access