PBx

PBX stands for private branch exchange, a private telephone switching system used within an organization. It interconnects internal phones and provides access to external networks via trunk lines or SIP trunks. By routing calls, a PBX enables extension dialing, call forwarding, voicemail, and other features without direct lines to the PSTN.

Traditionally, PBXs were electromechanical or electronic systems installed on-premises. With the growth of IP networks, IP-PBXs and hosted or cloud-based PBXs became common, offering software-driven call routing and reduced local hardware. Some organizations still use traditional KSU-style PBXs, while many adopt hybrid setups that combine analog trunks with IP extensions.

Typical PBX architectures connect to internal extensions and to external networks via trunk interfaces such as PRI/BRI, analog lines, or SIP trunks. In IP-PBX and hosted systems, voice traffic traverses an IP network using SIP or other protocols, while on-premises systems mingle digital signaling with traditional TDM.

Core features include automatic attendant or IVR, voicemail and unified messaging, extension dialing, call routing and hunting groups, call queues, conference bridges, and mobility options such as softphones and mobile apps. A PBX may support integration with CRM systems, fax gateways, and presence information.

Benefits include lower telephony costs through internal routing, scalability for growing staff, feature-rich call handling, and improved management. Drawbacks can be higher initial capital expense for on-premises systems, ongoing maintenance, and reliance on network availability. Cloud or hosted PBXs can reduce capital costs but raise ongoing subscription expenses and data security considerations.

Security considerations include protecting against toll fraud, secure trunking, and defending against SIP signaling vulnerabilities with encryption, firewalls, and proper network segmentation. PBX ecosystems generally rely on PSTN signaling, while IP-based PBXs use SIP, H.323, or compatibility with ITU standards to interoperate with other systems.

See also: telephony, VoIP, UCaaS.