Voiceflex SIP Trunks

SIP is the next logical step in the evolution of voice communications; providing voice calls over the internet provides additional features over and above traditional ISDN and analogue lines.

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  • Certified

  • Supported


  • Voiceflex is an advocate of SIP (Session Initiation Protocol)
  • Voiceflex uses its own SIP technology offering developed completely in-house
  • Winner of Most Innovative Channel Service


  • Paid
  • From £6 per month
  • Email sales@voiceflex.com



SIP Trunks are fast becoming the standard technology that allows businesses to make and receive telephone calls without the need for an expensive ISDN circuit. The Quality of Service has contributed to its increasingly widespread adoption.

The background

Session Initiation Protocol (SIP) is used to set-up and ‘tear-down’ telephone calls over IP networks, including the internet. It is this VoIP protocol that is most widely supported by modern devices, including IP-enabled PBXs.

A SIP Trunk is a facility provided by Voiceflex which allows the placing of outbound calls and the receiving of inbound calls using an internet connection instead of an ISDN or analog phone line. Calls to and from the PSTN are supported.

Voiceflex SIP trunks are designed to work with PBX devices and have been tested and approved by major vendors. We are able to supply PBX services to your office, as well as interface to existing PBX or SIP Gateway systems.


  • Signalling: SIP (RFC 3261). STUN supported but not required.
  • Voice: G.711 μ-law, G.711 a-law, G.729
  • DTMF: RFC 2833, SIP INFO
  • Fax: Inbound Fax-to-Email available. T.38 support planned.
  • Emergency services: 999 and 112 fully supported.
  • Security: Authentication by username and password with one-time encryption key.
  • Other features: CLIP, CLIR, network call-transfer, music on hold.
  • Telephone numbers: Geographic, non-geographic (geographic number porting also supported).
  • Required QoS: RTT < 100ms, jitter < 40ms, packet loss < 0.4% (see our networking products).
  • Provisioning: 5 to 30 minutes on all products when using web portal, 48 hours on all other orders (except number ports, 30 days).
  • Channel: Distributors, Resellers and Dealers. White label options available.
  • Support: First-line support by reseller, second and third-line support by Voiceflex.


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Support Details

Voiceflex offer support to all Channel Partners and end users. Voiceflex support is available 08:30 til 18:00pm monday to friday unless requested other wise. Voiceflex support can be contacted via email support@voiceflex.com or 02033016000. 

Support Contact Information



Voiceflex was established to provide advanced IP telephony (VoIP) services for UK businesses. Bringing the reliability of the Internet to the telephone, Voiceflex is an advocate of SIP (Session Initiation Protocol) - the latest technology allowing voice calls to be made over the Internet. Voiceflex uses its own SIP technology offering developed completely in-house, to provide low cost, ISDN replacement lines that provide the best possible call quality, inexpensively with the flexibility that comes from using the Internet. 

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