Private Branch Exchange (PBX) is a private telephone switchboard dedicated to a single business or office, as opposed to a Public Switched Telephone Network (PSTN), which is the interconnected voice-oriented public telephone networks, both commercial and government-owned.

For enterprises that want to make full use of their IP-PBXs and communicate via IP outside of the enterprise, a Session Initiated Protocol (SIP) Truck provided by an Internet Service Provider connects them to a traditional PSTN network. This interconnection of VoIP private and public domains allows for cost-effective unified communications solutions for streaming media, audio and video conferencing, along with other VoIP services.