PBX (Private Branch Exchange) is a private telephone system used by an organization to manage internal and external phone connections. It allows employees to call each other using internal extensions, make and receive external calls, transfer calls between users, and use additional business telephony features.
Modern systems that operate primarily over IP networks and use VoIP are commonly referred to as IP-PBX systems.
How PBX Works
A PBX sits between an organization’s internal telephone infrastructure and external telecommunications networks. Employees connect to the system using phones or software clients and are assigned internal extension numbers.
For example, a company may use extensions 911, 912, and 913. If the employee at extension 911 calls extension 912, the call is handled by the PBX itself and typically does not enter the public telephone network.
If an employee calls an external number, the PBX routes the call through the company’s connection to a telecommunications provider. An incoming call follows the reverse path: the provider sends it to the corporate telephone system, and the PBX routes the call to a specific employee, a group of agents, an interactive voice response system, or another destination according to predefined rules.
As a result, users do not need a separate external telephone line for every internal extension.
Main Functions of a PBX
The available features depend on the specific platform, but modern PBX systems typically support:
- Internal extensions and call routing.
- Call transfer and forwarding.
- Call queues and user groups.
- Interactive Voice Response (IVR).
- Conference calling.
- Voicemail.
- Call recording.
- Call handling rules based on time, number, or other conditions.
- Integration with CRM systems, contact centers, and other business applications.
A PBX also enables centralized management of an organization’s telephone numbers and communication policies.
Traditional PBX and IP-PBX
Traditional PBX systems were designed for conventional telephone lines and analog or digital telephone sets. They required dedicated telephony infrastructure.
An IP-PBX transmits voice and signaling information over IP networks. IP phones, softphones running on computers and smartphones, and other compatible devices can connect to such a system.
An IP-PBX often uses SIP (Session Initiation Protocol) to manage VoIP calls. The voice traffic itself is typically transmitted using RTP.
An IP-PBX can be deployed as a physical appliance, software running on an organization’s own server, or a virtual machine.
On-Premises and Cloud PBX
A telephone system can be deployed within a company’s own infrastructure or provided as a service by a third-party provider.
With an on-premises PBX, the hardware or software platform is managed within the organization’s infrastructure, for example on a server located in an office or data center. The company is responsible for configuration, updates, redundancy, and maintenance, or can outsource these tasks to a service provider.
A hosted PBX, or cloud PBX, runs in the service provider’s infrastructure. Users connect to it over an IP network, while the provider maintains the server-side infrastructure. This approach allows organizations to use business telephony without deploying their own PBX.
PBX and SIP Trunk
A SIP trunk is commonly used to connect an IP-PBX to a telecommunications provider. It enables external telephone calls to be transmitted over an IP connection using SIP.
A PBX and a SIP trunk are different components of the infrastructure. The PBX manages corporate telephony and call routing, while the SIP trunk provides connectivity between the telephone system and the telecom provider.
For example, a company may have 100 internal users connected to a single IP-PBX and use a SIP trunk to make and receive external calls.
Where PBX Is Used
PBX systems are used in offices, contact centers, hotels, healthcare facilities, support centers, and other organizations that need centralized management of telephone communications.
A modern IP-PBX can also bring employees from different offices and remote workers into a single telephone system. A user can have an internal corporate extension regardless of whether their device is connected to the office network or accesses the telephone platform remotely.