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This article is part of our Essential Guide: The basics of SIP trunking explained

SIP trunking vs. VoIP: What's the difference?

Telephony expert Jon Arnold explains SIP trunking versus VoIP and how each technology is used.

What's the difference between Voice over Internet Protocol (VoIP) and Session Initiation Protocol (SIP) trunki...


SIP trunking and VoIP are the cornerstones of IP telephony. Together, they provide the foundation for new capabilities that save money and make employees more productive. VoIP is likely more familiar to readers and refers to the IP-based version of legacy telephony.

As such, VoIP is a service, like dial tone, that businesses subscribe to just as they would with a time-division multiplexing (TDM) service. The main difference is that VoIP calls run over a data network, whereas TDM calls run over a circuit-switched network that is dedicated solely to telephony.

SIP trunking shares similar parallels to VoIP, where the equivalent in the legacy world would be PBX trunks. SIP trunks differ from PBX trunks in that they carry all forms of media, not just voice. Being designed for use in a data network, SIP trunks transmit packets, which could carry voice, data or video. This makes them more flexible than PBX trunks, as well as more cost-effective, since voice can be added to this channel, reducing the need for legacy connectivity.

SIP trunks are the physical connections that provide voice service to your phone system. PBX trunks only carry voice. They are expensive and inflexible but very reliable. VoIP can be routed over PBX trunks via a media gateway, but to take full advantage of VoIP's capabilities, businesses will deploy SIP trunks and typically route these calls via a session border controller.

This was last published in November 2012

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