SIP Trunk


Session Initiation Protocol (SIP) trunking is a Voice-over-IP (VoIP) technology that allows a company to route multiple VoIP calls through a single internet connection.

SIP trunks also have a distinct benefit of being able to fully integrate with legacy IP-PBX systems, therefore preserving investments in existing office-based PBX (telephone switch) hardware.

Communications as a Service (CaaS)

Delivered within the broader remit of Communications-as-a-Service (CaaS), SIP trunk providers typically offer a lower call costs when compared to traditional ISDN or national telephone carrier charges.  This is brought about by passing-on savings made at the carrier and call termination level where carrier IP networks have a much greater potential for efficient allocation of bandwidth and network capacity for voice calls than traditional voice (TDM) networks.

There are many compelling reasons to subscribe to a SIP trunk provider . Below we list out some key points:

  • Legacy system interface

If your organisation has already invested in an on premise PBX (telephone) switch, a SIP trunking solution will enable you to maximise savings on call charges and the cost of renting dedicated telecom circuits (ISDN) for your office(s).

  • Make and receive multiple calls

A SIP trunk allows for multiple calls to be sent and received concurrently, great for small office environments that historically may have been limited by ISDN2 TDM circuits.

  • Convert SIP calls to analogue

Using a simple SIP convertor you have the option of converting your SIP trunk calls to analogue, preserving any investment made in legacy systems.

  • Disaster recovery

You may not wish to migrate onto a SIP trunking service due to contractual obligations with your ISDN provider. A SIP trunk may be used to provide a secondary circuit in the event your primary ISDN service fails. As the service is internet based this would ensure that your business functions in the event of a line failure.

  • Non-Geographic Numbers

A SIP trunk allows you to use telephone numbers which aren’t necessarily representative of your location or tied to your traditional voice carrier.  This also makes for easier re-routing of number for call-overflow or in disaster recovery (DR) scenarios.

SIP trunks offer many compelling advantages over traditional ISDN based service both in terms of costs and flexibility although there are a number of key areas of consideration required before deployment.

  • Legacy hardware integration

A general rule for integration is that the majority of PBX systems manufactured after 2008 are already SIP trunk compatible. Always check your PBX (telephone switch) is compatible with a SIP service and discover costs of installation and purchase of a SIP-ISDN gateway, or SIP interface for your PBX.

  • Reliability and internet access

SIP trunks when installed correctly over a fixed circuit are of excellent quality (subject to compression and codec) and ultimately offer excellent cost and operational advantages. However it is imperative to ensure that your internet connectivity is compatible with the providers recommended specifications.  In some cases it may be possible to get a direct connection to your SIP trunk provider and this may be offered with enhanced SLAs.  If this is available we would recommend following this course of action for your primary sites.

  • A SIP trunk is not a PBX!

A SIP trunk is a method to deliver calls to your internal PBX system or VoIP gateway it does not provide call features. A simple analogy to use is that the SIP trunk is the routing mechanism to deliver and receive your voice calls.

  • Integration with VoIP software

Always check with the provider of your VoIP software compatibility and settings for your SIP trunk.  Once this is confirmed always do a test to confirm call quality and routing.

At Compare the Cloud, we’re here to help you get started and to help you identify suitable SIP trunk providers to evaluate and work with.  Take a few minutes to tell us about your company in our Cloud Comparison Tool, and we’ll present you with some informed options – and help you take full advantage of what a SIP trunk service offers.

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