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Home > Business SIP Explained

Business SIP Explained

SIP Trunking & Hosted Phone Systems

From £4 per SIP trunk or £6 per telephone extension

Spitfire SIP Trunking is one of the leading SIP Trunks services available in the UK – bringing reliable and flexible VOIP to your office via Ethernet or DSL circuits. Spitfire Cloud 3CX harnesses the power of 3CX, the world’s leading windows based phone system. It offers significant cost savings and improved flexibility compared with an on-premises phone system, and brings an excellent range of Unified Communication features.

Business SIP Explained

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Residential VoIP v SIP

While residential VoIP generally means services such as Skype, business VoIP users require the same quality as ISDN30e.


ISDN30e quality is delivered by ITSPs (Internet Telephony Service Providers) who deliver VoIP services using SIP – Session Initiation Protocol.


What is SIP?

SIP is the protocol that controls the call, setting up the call and closing it down. The actual voice media uses another protocol, RTP (Real Time Protocol), sent as normal IP data packets, so a conversation is broken down into thousands or tens of thousands of tiny IP data packets, each one carrying a fraction of a seconds worth of conversation.


Call quality guarantees

The key to SIP call quality is that every single IP data packet, with its millisecond’s worth of conversation, arrives in the correct order and without any delay, so every packet arrives after the previous one, in a constant stream. To ensure this happens 100% of the time you need an Internet connection that has guarantees covering both up time/availability and quality, in terms of packet loss (packets never arrive), jitter (varying time delay between packets) and delay (how long each packet takes to be delivered).


A key issue is that if the Internet Service Provider (providing your Internet connectivity) cannot guarantee that your traffic will be forwarded onto your ITSP (providers of the SIP Trunks) without further delays, the data may be passing through multiple networks before it gets to your ITSP. Even if interconnects are in place, these may change and your service may suddenly deteriorate.


Using an ITSP that is also an ISP (Internet Service Provider), such as Spitfire, means that the data packets are delivered directly to the voice network and assembled as a telephone call without any extra delay. This guarantees that you have a single provider who, with the right circuit, can guarantee call quality and ensure ISDN30e call quality, within the M25.

Suitable data circuits


Standard ADSL is NOT suitable to carry voice – the packets arrive with varying delays, sometimes slowly, sometimes all in a rush, sometimes correctly. When using ADSL, most calls might be fine – but if the network suddenly gets congested, call quality suffers, or the call cuts out altogether.


Without doubt, the best circuit to use is a Business Ethernet circuit (EFM/Copper Ethernet or Fibre Ethernet). This is ideal for any business requiring 10 simultaneous calls or more. The same circuit can also be used for data, avoiding the need for a separate leased line or other Internet connection.


SDSL and SDSLM are also suitable for voice, and will support up to 15 simultaneous calls. Spitfire also offer a Voice Approved ADSL circuit, which will support up to 4 calls, at a cost of just £21 per month.


The key with all these circuits is that they are designed to carry voice – meaning we can guarantee the call quality on Spitfire SIP Trunks over Spitfire Internet circuits.


Number Provision

Wherever your office is, wherever your calls will be answered, we can allocate you any area dialling code you want. We have our own numbers allocated on all UK dialling codes, which gives us (and you) control over your numbers, rather than relying on a third party as many smaller ITSPs have to do.


You can therefore have a mixture of London and Glasgow numbers delivered to an office in Birmingham – as well as local numbers. Ideal if you want to give a local presence to your business.


What’s more, any BT number, and numbers from many other providers, can also be ported to our network – allowing you to retain your current phone number when moving premises.


Visit our Business Sip Trunks page to discover the benefits of Spitfire’s service, or talk to our expert consultants on 0800 319 6363.

Back up Options

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SIP is ideal as a Disaster Recovery (DR) solution , with multiple levels of resilience. The first level of resilience is the ability for SIP calls to be routed to a different IP address, i.e. a backup Internet circuit, or a circuit at another location.


The second level of resilience is to be able to divert calls to a back up ISDN30e or local telephone numbers – this can be done on a DDI to DDI basis. Most users will not even know there is a failure. As an alternative, or if you don’t have back up lines on site, we can forward calls to multiple external numbers. Again, this can be done on a number-by-number basis, so individual DDIs can route to home or mobile numbers on a person by person basis. This is ideal if you are concerned about your main site being inaccessible or inoperative.


Any number on the Spitfire network can be forwarded to any other location – as long as it is connected via a SIP Trunk or a normal telephone line.


The ability to provide Spitfire SIP Trunks and Off-Net SIP Trunks via our wholesale partner, Gamma Telecom, can be used to introduce another level of resilience into the solution.


This DR flexibility is just not available with ISDN30e – please discuss with us your requirements and we will construct a DR plan that suits you.

IP Compatible Phone Systems


The following systems have completed full Interoperability testing and are certified for use with Spitfire SIP Trunks. The systems are also tested with each new software release. Spitfire are able to recommend partners who hold manufacturers accreditation and are also authorised Spitfire Partners, allowing you to benefit from an integrated and co-ordinated service delivery.


Spitfire produce configuration guides for all the systems listed below, allowing you or your system maintainer to easily configure your phone system for Spitfire SIP Trunking. We also offer technical support to help you or your maintainer in the event of any issues.


Please note that in general you will require a competent technical person to set up a PBX to work with any SIP Trunking provider. If you require on site support or more in depth assistance, Spitfire can provide this on a chargeable basis either via our Support and Installation Team or via one of our accredited partners.

Phone System Config Guides

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Click on the links for a downloadable PDF.


Please note that this list is being continually updated.


If a system isn’t listed, please call us on 020 7501 3333 or email an enquiry.

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