# What is a Private Branch Exchange (PBX)?

PBX, short for *Private Branch Exchange*, is a phone system in an enterprise that manages incoming and outgoing phone calls as well as an organization's internal communications.

## PBX Basics

Modern PBX systems help make an organization's communication more streamlined and robust. At the most basic level, a PBX connects communications devices such as hubs, switches, telephone adapters, routers, and phone sets. Size and complexity vary with each PBX. There are at-home PBX systems that upgrade a traditional phone line to cloud-hosted programs as well as complex and expensive corporate communications systems owned and operated by an individual enterprise.

*Note*: PBX is sometimes written PABX (*Private Automatic Branch Exchange*). While the first PBX systems required a human operator, almost all modern PBX systems are now automated. 'PBX' is still the term most commonly used to refer to this kind of system.

## The Many Functions of a PBX System

A PBX system makes it affordable to use more than one phone line in an organization. Managing incoming and outgoing calls, a PBX makes it possible to split a single phone line into several private lines identified by extensions (usually assigned 3 or 4-digit numbers). Not only does this allow a customer to reach anyone in an office through a single phone number, but it also grants the group free internal phone communication since multiple unique phone lines are no longer a requirement. PBX systems also empower [VoIP communication](/docs/glossary/what-is-voip).

PBX systems perform four main call processing duties:

* Establish connections between the phone sets of two users.
* Maintain connections as long as the users require.
* Disconnect a connection per the user's requirements.
* Provide information to the organization for accounting and analytics.

While it's safe to assume that all PBXes offer the above features, most modern PBX systems provide a whole host of other calling features and capabilities (though each PBX may differ in which features they offer). Some commonly provided PBX features include:

* **Call management** via call blocking, call forwarding, call logging, call transfer, and call waiting.
* **Streamlined customer experience** with call recording, voicemail, [IVR](/docs/glossary/what-is-ivr) (*Interactive Voice Response*) and [DID](/docs/glossary/what-direct-inward-dialing-did) (*Direct Inward Dialing*). Custom greetings, welcome messages, and hold music are other useful customer-facing features often included with a PBX.
* **Internal communications** via conference calls and internal extensions.
* **Local connections** allowing users to have local numbers in cities where they're not physically present, enabling 'virtual offices' anywhere.

## What is IP-PBX?

The arrival of IP telephony ([VoIP](/docs/glossary/what-is-voip)) changed a lot about PBX. While PBX systems were initially analog and built on telephone lines and switches, IP-PBX systems use VoIP technology and IP networks to channel calls.

IP-PBX is the preferred flavor of modern PBX, switching calls between a VoIP user and a traditional phone user or between two standard phone users. Not only are IP-PBX systems feature-rich, but they also don't require separate networks for voice and data communications. With IP-PBX, a single user has access to the internet, VoIP communications, and traditional phone communications through one line. Most, though not all, PBX systems today are IP-PBX.

## What is a Hosted PBX?

Also known as Virtual PBX, a Hosted PBX delivers PBX functionality as a service through the cloud. Various companies offer PBXes at a monthly cost, and all users need are their telephone sets and routers. While Hosted PBX systems generally cannot be tailored to more complex business needs, they are a great option for those running a small business due to their relatively low cost and the fact that they don't require any upfront investment.

## Incorporating Twilio with your PBX

If your business uses a PBX and you're interested in integrating your system with Twilio, we're here to help!

![Diagram showing connection from customer to Twilio via PBX and SIP, delivering calls over PSTN or WebRTC.](https://docs-resources.prod.twilio.com/5670108b3c25ba48a21bcdafd49bf1545396c75443a34c94fc09716a5da39f8b.png)

Twilio's [Programmable Voice SIP](/docs/voice/api/sending-sip) allows you to use your existing SIP communications infrastructure to initiate SIP sessions with Twilio. You can also [add WebRTC to your PBX call center](https://www.twilio.com/blog/add-webrtc-to-your-pbx-or-call-center-in-5-minutes-html) or [connect your PBX to Twilio's Elastic SIP trunking](/docs/sip-trunking/sample-configuration).