SIP Trunks

SIP trunks offer a more flexible and lower cost alternative to ISDN for inbound and outbound voice calls.

SIP trunking is a standards-based replacement for traditional ISDN. It connects your PBX to the PSTN via broadband, ethernet or private circuit into our national network.

Compared to ISDN our 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.

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.

Our SIP trunks work seamlessly with each of the leading IP PBX brands in the UK market. View the PBX vendors we work with.

Any business with a PBX can switch to our SIP trunking service and immediately benefit from a more flexible, less expensive and more resilient phone service.

Try our SIP vs ISDN cost calculator right now and see for yourself.

Don’t have a PBX? Why not consider one our Cloud Phone Systems.


If you’re offline you can’t work effectively. Out of touch means out of the picture, and worst case out of business. With 99.999% availability, our network keeps you working and your workforce productive. For contingencies, our business continuity and disaster recovery services help keep disruption to a minimum.


Changing working demands mean you can no longer afford to be tied to a desk. Go wherever you want, work wherever you want. Your calls can follow you. You’re available when you need to be, with powerful call handling for when you can’t. And with clever call features on our Horizon platform that extend to your mobile devices, accessibility extends virtually everywhere.


You run a lean business but you still want the option to scale up when demand dictates. With Flip you can. Add new extensions quickly and easily. Pay only for what you need, when you need it. Futureproof your business with a phone system that can support your business needs now and in the future.


Take control with simple management via our customer portal. We give you the online tools, billing data and stats you need to manage your telecoms your way. You’re always in control of costs, calls and usage. And access to enhanced call statistics and real-time reporting ensures you can make informed operational decisions, quickly and when it matters most.


We’ll ensure your telecoms never goes stale or loses its edge. Unlike other providers saddled with legacy assets to protect we’ll never leave your telecoms in a time warp. We constantly refresh our infrastructure and network services to give you options for the future, helping keep your business ahead of the pack.


Forget punishing line charges and proprietary hardware. Flip’s cloud voice services help simplify your bill with inclusive minutes to help you budget better. Move from large, lumpy capital expenditure to a predictable opex model, where everything – including handsets – is included in one fee per user, per month.

Case Study

Lintbells has been making premium quality, natural supplements for pets since 2006, when John Davies and John Howie discovered a shared passion: the difference great nutrition can make to pets’ lives.

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Looking for a telecoms system that can drive growth?

Download our SIP Trunking costs and savings guide

Download our eGuide

Save money on call costs and line rental

SIP delivers less costly, more reliable connectivity than ISDN over an IP connection. It can save up to 50% on line rentals and 25% or more on call costs.

Free internal calls and great features

Perfect for medium or large organisations with multiple sites, not only will you save on your line rental and call costs, but you’ll get free internal calls within your organisation and a whole host of great features.

Scale up and down when you want

SIP trunks are ideal if you are moving, upsizing or downsizing and want to keep your existing numbers. Scale up lines during busy periods and back down again afterwards. You can add lines, take them away, split calls at any time. You are always in control.

Built-in Business Continuity

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

Compatible with Skype® for Business

Our SIP trunks are integration tested and approved to interoperate with Microsoft Skype® for Business, allowing you to directly connect to the platform to make external calls.

Quick Installation

With traditional call breakout, such as ISDN taking up to 8 weeks to install, SIP can be deployed within hours.
Savings Saves up to 50% on line rentals and 25% on calls Expensive
Flexibility You’re in control. Add lines, take them away, split calls – it’s up to you Sorry, can’t do flexible
Control Choose how and where you want calls delivered. Make changes instantly Changes take months. At the mercy of suppliers
Future Well mapped out and ready for Voice/Data convergence Going nowhere fast
Contingency Handles emergencies with ease. Reroute calls to backup sites in seconds Hope nothing bad happens

SIP solutions for your business

Number flexibility

Flip SIP Trunks support business relocations by enabling a geographic number to be retained without call forwarding costs. This ensures business continuity and reduces the costs of having to change company stationery, listings and advertisements. As a leading UK communications provider we can also supply out-of-area geographic number ranges to make your organisation appear local, even without a physical presence.

Business-grade voice and internet convergence

With our SIP trunks delivered over Flip Broadband or Flip Ethernet, you’ll have a high-quality voice and data service from just one connection, making for significant cost savings.

Line and PBX estate rationalisation

For businesses with several locations, our SIP trunking service provides ISDN line rationalisation – the ideal way to reduce the number of PBXs requiring maintenance.

Business continuity

Businesses of all sizes need a business continuity plan for telephony. Combine our SIP trunks with our Inbound service and your callers will always get through. Set automated call plans or simply divert published numbers to backup locations or mobile devices – instantly and from anywhere.

Affordable resilience

If you’ve ever had to deal with a service outage you’ll know that speed of response is key. Our active/standby and load sharing designs are engineered to cope with any situation, so maintaining telephony uptime. Whether you need to keep your businesses running in an emergency without the high cost of call forwarding or simply need to balance call loadings between sites during peak, Flip SIP Trunks will provide a business grade solution.

Free UK calls and fraud protection

Our SIP trunking service provides free call termination to UK fixed destinations, including 01, 02 and 03 number ranges. In addition, SIP provides free calls between connected sites, including international locations. CallGuard protection from unusual usage is also included free of charge with all of our SIP trunks.

Manage your entire inbound number estate through your SIP trunks

SIP Trunk Call Manager is a brand new solution to the market and encompasses all of the benefits of SIP combined with a unique web-based portal and app service that enables you to take full control of all aspects of your inbound calls.

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

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 Flip 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.

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

5 common concerns about moving to SIP

Download the eGuide

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 Flip SIP Trunks.

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.

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, Flip SIP Trunks can do this and at no extra charge to you.

What if we’re moving office?

Number flexibility with SIP trunking means that you can keep your existing number, even when moving out of the area, saving you costs on printed company literature and stationery whilst maintaining the perception that you still have a presence in the area you’ve moved from. We can easily transfer your old or existing numbers too so that you can avoid expensive Remote Call Forwarding (RCF) or call diversion charges.

With CLI flexibility you can also decide which number you want to display on a call by call basis – great for contact centres.

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 Flip 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.

Does SIP work with other business networks and applications?

Flip SIP Trunks are integration tested and approved to interoperate with Skype® for Business, allowing you to directly connect to the platform to make external calls.

We’re also a Janet-connected partner with an interconnect to provide voice services through Flip SIP Trunks to the Janet community. Janet is the network dedicated to the needs of research and education in the UK, connecting institutions to each other, as well as to the rest of the world.

Mobile Solutions

With no allegiance to any particular carrier but access to unbeatable business package, we are ideally placed to offer superb mobile solution with first class support.

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Inbound is a feature-rich telephone service with online access to a full range of call routing, monitoring and management tools to empower business and drive customer service.

Find out more

Cloud Phone Systems

Complete business communications services that provide an extensive range of fixed and mobile telephony capabilities via easy-to-use web portals.

Find out more

Use our Quick Quote Tool below to help us find the best solution for your business. Once we have received your enquiry one of our telecom consultants will contact you within 2 working hours.
Which service are you looking for?

What type of phone lines does your business currently have?

How many lines do you currently have?

Are you looking for a new phone system?

How many Employees in your business?

How reliant is your business on internet access?

Rate your businesses daily internet usage (include any hosted services as internet usage)?

Do you require a dedicated non-contended broadband connection?

Do you currently have a phone system or plan on installing one in the future?

How many mobiles do you require?

Are you interested in integrating your mobile phones with your office systems?

Do you have a specific network that you prefer?

How many phones do you require?

How many lines do you require?

Do you have more then one office or remote workers?

Which additional features would you like?

Contact Details

Please provide your contact details below and one of our telecom consultants will contact you with a personalised quotation within a few hours.

Rebecca Davies, Lintbells