SIP Trunking

Spitfire are a leading business Internet Telephony Service Provider (ITSP), as well as being a business telecomms operator and Internet Service Provider.

Spitfire are one of the only ITSPs able to offer a complete end-to-end service, via their own networks that covers all aspects of VoIP. The result is a service that actually works and delivers business quality VoIP.

For further information about how Spitfire can get SIP trunking working for your business please enquire here or telephone 0800 319 6000.

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Business VoIP & SIP

While VoIP covers many different things, business ITSPs almost all use SIP – this is seen as a viable alternative to traditional ISDN. SIP Trunks refer to individual SIP sessions and are in effect sold like ISDN30e lines, so 10 SIP Trunks will allow you to have 10 simultaneous calls via your SIP provider.

SIP (Session Initiation Protocol) is the protocol that controls the call, this includes setting up the call and closing it down. The actual voice media uses another protocol RTP (Real Time Protocol), sent as normal IP packets. The IP traffic requires an internet connection to travel over and this is where the fun begins, as the required bandwidth is not normally provided by your ITSP. So just because you have ordered 10 SIP trunks doesn’t mean that you will have enough bandwidth to make 10 calls.

Spitfire are the only business ISP who are also an ITSP and an Interconnected operator. As well as offering SIP Trunks we have a leading hosted solution – Spitfire SIP Communicator™ – for more information on this please Click here.

Number Provision

We have numbers allocated on all UK dialing codes – allowing you to have the dialing code you require. In addition to this, any BT Wholesale number can be ported to our network – allowing your clients to retain their number when moving premises.

Backup Options

Spitfire are committed to ensuring you have the highest level of uptime – as well as incorporating a backup ADSL circuit with our SDSL service, we are able to divert calls to individual mobiles or other numbers as required. This can integrate with some back up analogue or ISDN lines, or to a DR site as required.

Cost Comparison – Spitfire SIP trunks and BT Retail ISDN30e

For a customer with 10 ISDN30e lines and 100 DDI numbers, Spitfire SIP Trunks are 48% less expensive – with a wide range of back up options:

  • One year BT Retail ISDN30e with 10 channels and 100 DDI quota plus CLIP - £190.80 per month.
  • Spitfire SIPStream SDSL 2Mb 1:1with back-up ADSL circuit, 10 SIP trunks and 100 DDIs - £99 per month.
  • Saving over £90 per month, with guaranteed ISDN30e equivalent quality.

What is SIP trunking?

  • SIP trunking provides access to and from the Public Switched Telephone Network (PSTN) over the Internet Protocol (IP).
  • Businesses use SIP trunking to provide cost effective telephone service to their offices through a PBX or individual users in remote locations. It also can enable a business to move existing number ranges to an out of area location.
  • Please click on the link to learn more about ASTERISK.

Why Spitfire?

  • Spitfire runs its own core network infrastructure we do not resell anyone else’s service.
  • We can provide telephone numbers from geographic areas all over the UK and route them to your SIP service, enabling you to have a virtual office anywhere.
  • Our SIP trunking has flexible options for providing backup to ISDN lines and vice-versa.
  • Spitfire can provide all the components necessary to connect your telephone system via SIP including quality IP access and gateways.
  • We have experience of making SIP solutions work from end to end.
  • With our dedicated test facility we have tested our service with a variety of manufacturers’ SIP devices including Asterisk, Avaya, IP Cortex, 3CX, Epypi, Panasonic, Patton, PBXnSIP, Solutions 11, Trixbox, Vegastream, Snom, Polycom, Linksys and Counterpath.


  • SIP Internet Bearer Circuits - Spitfire Business 2Mb 1:1 SDSL – from £49.00 per month(for use with Spitfire SIP trunks)
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