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

SIP trunking and voice over IP share some similarities, but they have key differences in origin and functions. How well do you understand these telephony cornerstones?

What's the difference between voice over Internet Protocol (VoIP) vs. 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.

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

VoIP is often associated with phone systems -- and for good reason. In enterprises, legacy telephony was defined by phone lines connected by a trunking service -- namely T1 lines -- to a premises-based PBX system. Since the 1970s, Private Branch Exchange has been the standard telephone network architecture in business environments, at least until VoIP came along.

Similarities of SIP trunking and VoIP 

With the advent of VoIP came the IP PBX, which has now become the successor for premises-based phone systems. Similarly, SIP trunking has emerged as the successor to T1 for the trunking service to support enterprise telephony.

The other similarity is both are protocols, which define the standards for communications over IP networks. For SIP trunking, the acronym stands for session initiation protocol, and this standard applies to a wide range of communications applications beyond voice, including instant messaging and video. VoIP is more specific, where the Internet Protocol is concerned only with how voice is transmitted over IP networks.

Differences between SIP trunking vs. VoIP

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, as voice can be added to this channel, reducing the need for legacy connectivity.

Furthermore, once voice is added to a SIP trunking service, the enterprise can support all communications applications end to end, and that provides the ideal environment for unified communications (UC). This moves telephony off the voice-only island of TDM and into the integrated world of UC, where it can be more effective in supporting the needs of today's workers.

While these two elements work very well together, the differences of SIP trunking vs. VoIP need to be understood. 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.

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What is the telephony setup in your company, and what roles do SIP trunking and VoIP play in it?