What is SIP trunking and how does it work?

SIP trunking allows businesses to make and receive local or long-distance phone calls and other digital communications over the internet, instead of traditional phone lines.

SIP stands for Session Initiation Protocol, which is a signalling protocol used for establishing, maintaining, and terminating real-time sessions that involve video, voice, messaging, and other communications applications and services.

SIP trunking works by establishing a virtual connection between the business’s private branch exchange (PBX) and the service provider’s PSTN network over the web.

Learn more about how our voice, connectivity and mobile products can help you achieve your comms goals by contacting Gamma or one of our specialist partners.

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Personalise how you communicate with Gamma SIP Trunks.

Gamma SIP Trunk Call Manager

SIP Trunk Call Manager takes SIP beyond a connectivity service into a world of multi-feature applications, putting you in control.

SIP Trunk Call Manager provides you with all the benefits of Gamma SIP Trunks together with a centralised inbound call management service with a host of features, accessed through an easy-to-use web portal and mobile app. SIP Trunk Call Manager offers powerful business continuity as standard, giving you the ability to manage your entire number estate and all aspects of your inbound calls. It enables you to consolidate all your numbers onto a single platform, meaning that you’ll never miss a call.

The benefits of Gamma SIP Trunk Call Manager

  • Enhanced SIP service – gives you full and direct control of your inbound numbers.
  • Tailored business continuity – never miss a call.
  • A direct ISDN replacement – save up to 50% on line rental and up to 25% on calls.
  • 01, 02, 03 and 08 termination on subscribed Gamma SIP Trunks – no number translation.
  • One point of contact providing your SIP trunks and call management via one simple solution.
  • Easy management via an app and web portal.
SIP Trunk Call Manager

PCI DSS compliance

The Payment Card Industry Data Security Standard (PCI DSS) is the proprietary information security standard defined by the major card companies to help combat fraud and protect consumer card data.

Its members include Visa, MasterCard, American Express, Discover and JCB. PCI DSS applies to all organisations that store, process or transmit cardholder information, from any of these members’ cards. The type of annual assessment required varies according to “level”, which is defined according to the volume of payment transactions that are handled.

With 327 controls to consider in total, it is vital to ensure you have them all ticked. The effects of being non-compliant can be disastrous for a business due to the very real financial risk of opportunistic agent fraud and the associated reputational risks. Some of these include:

  • Compromising your customers’ payment data.
  • Loss of customers, damage to your brand and reputation.
  • Expensive lawsuits and insurance claims.
  • Substantial payment card user and Government fines.

Gamma SIP Trunks with Semafone’s award winning solution

We’ve partnered with Semafone to offer a flexible and cost-effective telephony solution to ensure your organisation remains compliant when taking Cardholder Not Present (CNP) payments over the phone. Combining our market leading Gamma SIP Trunks with Semafone’s award winning solution to protect your business from risk.

It’s quick to install, helps to improve agent productivity by reducing Average Handling Times and supports connections ranging from two channels to an unlimited number.

PCI DSS Compliance

SIP Call Guard Service

Protect your business from costly phone hacking and misuse

Hacking and abuse of business telephony is becoming as big as credit card fraud. With this type of abuse, the cost of the calls falls to your business. But with our CallGuard service your business is protected from such activity.

  • Your calls protected from unusual usage.
  • Low cost for peace of mind knowing your calls are protected.
  • Avoid any unwanted expensive bills.
  • Can be applied to all of your existing numbers.
  • Tailor your protection – opt numbers in and out as required.
SIP Call Guard Service


  • How many SIP trunks do I need?

    Most businesses will only require one SIP trunk, because they only operate one PBX system. However, this SIP trunk will be comprised of multiple channels. To decide how many channels you require, a good rule of thumb is to configure one channel per every simultaneous call that is generated by the SIP gateway. The good news is, there’s no limit to how many channels you can place in a trunk – and your provider can add or remove channels to meet your needs.

  • How many calls can a SIP trunk handle?

    Technically speaking, there is no maximum number of channels that can be assigned to a SIP trunk, which means your service will be able to handle a limitless number of calls. However, each channel will require a certain amount of bandwidth, depending on the type and size of the data that is being sent and received across the network. This will need to be considered when planning and implementing your SIP trunking solution.

  • How much does a SIP trunk cost?

    Prices will vary depending on your channel capabilities and individual call charges (which are often calculated on a pence per minute basis). You can expect to pay between £50 and £150 for the monthly service rental, and between £3 and £150 per SIP channel. You will also need to consider the cost of connecting your phone system to SIP trunk services; installation fees typically start from around £300. Number porting facilities will incur extra costs.

  • What is the difference between SIP trunking and VoIP?

    The two technologies are closely related, but there are subtle differences. VoIP systems enable users to make phone calls via their broadband connection. SIP trunking can sometimes rely on VoIP when it comes to placing and receiving calls via the Cloud, as VoIP systems can only transfer voice data over the internet – but a SIP trunk can in fact transfer voice, text, or audio media, meaning it is capable of carrying more than one VoIP stream at once.

  • What if we are moving office?

    Number flexibility with SIP trunking means that you can keep your existing number, even when moving out of the area. This saves you costs on printed company literature and stationery while maintaining the perception that you still have a presence in the area you have moved from.

  • Can SIP protect the business in an emergency?

    SIP trunks can handle emergencies easily. They are inherently disaster recovery/business continuity ready, allowing you to reroute calls to an alternative location quickly and easily.

  • Does SIP work with other business networks/applications?

    Gamma SIP trunks are integration tested and approved to interoperate with Skype® for Business, allowing you to directly connect to the platform and make external calls. We are also a Janet-connected partner.

  • How scalable is SIP?

    The service can support connections ranging from two channels for small PBX-equipped businesses to an unlimited number of channels for large enterprises and contact centres. You can also add lines and take them away at any time.

  • What are the benefits over our ISDN setup?

    SIP trunking service is less costly per channel and more flexible in how and where geographic telephone numbers can be used. It’s also quicker to install and provision and, as standard, it delivers extremely robust business continuity.

  • What if we want to add or remove some of our lines?

    SIP trunks are valuable for businesses with seasonal voice capacity requirements where more lines can be quickly and easily implemented with no long term commitments, for example when running sales campaigns.

    Conversely, if your business has multiple sites with ISDN connections into each, using Gamma SIP Trunks you can reduce the number of ISDN connections that your business requires and also reduce the maintenance costs on all of your various PBXs. At the remote sites you are left with just the cost of an IP connection and the handsets that you need and you can then pull all the telephony traffic back into your centralised PBX.

  • What happens if our internet or power goes down?

    Should disaster happen, you can use SIP trunking to minimise disruption to your business. For example, if your London branch has to close in an emergency, and you need the calls routed to Manchester immediately, Gamma SIP Trunks can do this and at no extra charge to you.

  • What’s the quality like?

    From the moment we connect your SIP trunking service, you will enjoy all the reliability, resilience and quality that comes from working with the UK’s leading SIP trunk provider. We proactively monitor and manage quality of service across our entire network – it’s the underlying network quality that makes a real difference to SIP trunking. And as a consequence we are able to support both G.729 or G.711 codecs.

  • How resilient is SIP trunking?

    SIP trunking is now a mature service enjoyed by thousands of UK organisations and this alone is testimony to the reliability of SIP. For added reassurance while you switch to us, SIP trunking can even be used with existing ISDN connections to ensure the transition goes smoothly.

    Owning our own national network means we can identify and fix issues faster, through our advanced monitoring and diagnostic tools. It’s a combination which has helped us win many major UK Fixed Line Network awards in recent years and one of the reasons more and more businesses are specifying Gamma SIP Trunks.

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