A private branch exchange (PBX) is the phone system a business uses internally. It gives each person an extension, lets calls be transferred and parked, runs voicemail and the main-number menu, and connects the organization’s phones to outside phone lines so staff can make and receive ordinary calls. Traditionally a PBX was hardware in a telecom closet; today it may be software on a server, a virtual machine, or a service a provider runs for you.
At a glance
- A PBX handles calls inside the organization and connects them to the public phone network through outside lines.
- Common features include extensions, transfers, hunt groups, voicemail, auto attendants and call reporting, with details varying by system.
- Older PBXs connect to the phone company with analog or PRI lines; IP PBXs usually connect with SIP trunks.
- A PBX can be owned and run by you (on premises) or run by a provider as a hosted PBX or as part of UCaaS.
- End of manufacturer support, line retirements and maintenance costs are the usual triggers for replacing one.
What problem it solves
Without a PBX or a similar service, each desk would typically need its own line from the phone company, and calling a colleague would mean placing an outside call. A PBX lets an organization share a smaller number of outside lines across many phones, dial colleagues by extension, and route incoming calls to the right person or team. It also gives the business one place to set call handling rules: what callers hear after hours, which phones ring for sales, where voicemail goes.
For buyers today, the question is less whether to have a PBX and more where it should live and who should run it. Many owned PBXs were installed years ago, are maintained under a contract with a dealer, and depend on lines that carriers are repricing or retiring in many areas.
How it works
Call control. The PBX keeps a directory of extensions and a set of routing rules. When someone dials, it decides whether the call stays inside the organization or goes out to the phone network, and over which line.
Endpoints. Desk phones, conference phones, softphones on computers and mobile apps register with the PBX. Older systems use analog or proprietary digital phones wired back to the PBX; IP PBXs use phones and apps that connect over the data network.
Outside lines. The PBX connects to the public switched telephone network through trunks. These have historically been analog lines or PRI circuits, and on IP systems they are usually SIP trunks, often through a session border controller (SBC) that secures and normalizes the connection. Each user can have a direct number (DID) that rings straight to their extension.
IP PBX. An IP PBX is a PBX that carries calls as Voice over IP. Most systems sold today work this way. Many still include gateways or cards so analog devices, such as fax machines, door phones and paging, can stay connected.
Emergency calling. In the US, multi-line phone systems are subject to federal rules on direct 911 dialing (Kari’s Law) and on sending a usable location with 911 calls. Which rules apply depends on the system and when it was installed; other countries have their own requirements.
When it matters for buyers
- When the PBX reaches end of support. Without security patches and parts, a failure can mean a long outage.
- When the carrier reprices or retires analog or PRI lines that the PBX depends on, which forces a change to trunks or to the system itself.
- When maintenance renewals come due, especially across several sites with different systems.
- When staff work remotely and need their extension on a laptop or mobile phone.
- When consolidating after a merger or acquisition, since separate PBXs make transfers and dialing between companies awkward.
Questions to ask vendors
- What is the end-of-support date for our current PBX model and software version, and what does extended support cost?
- If we keep the PBX, can it connect to SIP trunks, and do we need an SBC or gateway?
- If we move to a hosted PBX or UCaaS, which features from our current system have no direct equivalent?
- Which of our phones, analog devices and paging systems can be reused, and which must be replaced?
- How will 911 calls send the right location for each user, including remote workers?
- Who handles number porting, configuration and training, and what does cutover look like site by site?
- What are the total costs over the contract term, including licenses, maintenance, devices and calling?
How it differs from hosted PBX
A hosted PBX does the same job, but the provider runs the system in its own data centers and you pay a monthly fee instead of buying and maintaining hardware or software. With an owned PBX, you control the system, its upgrades and its carrier connections, and you carry the maintenance, security patching and replacement cycle. Unified Communications as a Service (UCaaS) goes a step further by adding video meetings and team messaging to the hosted phone system.
If you are weighing whether to keep, replace or move your phone system to the cloud, our hosted PBX solution page covers how buyers compare the options.