How Centrex works?

A virtual Centrex service (also called Hosted PBX) uses the special SIP protocol to allow voice communications over a broadband internet connection. Our virtual PBX and PSTN call servers allow us to provide a single telephony solution for distributed organisations.

When a call comes in, whether it is from the PSTN (regular UK phone) or another virtual PBX user, it will arrive at our call routing server. The call server forwards the call to the relevant telephones, which then ring. You define the 'relevant telephones' using our control panel.

SIP works in such a way that once the call signaling has completed, the voice channels go directly between the two devices, i.e. the two end user telephones, or in the case of a PSTN call, our call server and your internet phone.

• For a full definition of every terms, see the Jargon Buster page.
  
• To get you started with VoIP technology, see our 'Knowledgebase' by clicking on Support.
  
• If you want more, see our VoIP FAQs by clicking on Support.
  

Can I integrate my existing phone line?

Yes, we are able to transfer both geographic and non-geographic telephone numbers from BT, plus a number of other SIP service providers such as Gradwell, Free World Dialup and VoIPTalk to our service.
Also, we are aware of solutions that would allow you to integrate larger numbers of existing phone extensions and ISDN circuits. Typically, our service could be integrated with any "SIP" enabled device. Contact us if you want to discuss this further.

What about NAT and security?

This is a bit technical, but don't worry. If in any doubt, please contact us to help you answer any questions you may have. So, here we go: because voice channels are set up independently of the call signaling, and because you may receive telephone calls from any other telephony user, it is somewhat difficult to firewall an internet telephony service and/or use it behind a NAT (network address translation) gateway (although not impossible).
If your computer is behind your ADSL router and has an IP address starting with 192.168, 10.0 or 172.16 then it is likely that you are using NAT and will have to complete advanced configuration to your firewall or router to use this service.
This means that where possible you will need a public IP address to plug your internet telephone into.
We recommend that you permit port 5060 to access your internet phone, as well as UDP traffic on ports 1024 up to 65000. Dependent on your device, you may be able to reduce this port range. The traffic received on these ports should be forwarded to your telephone.
We strongly recommend that you use a dedicated internet telephone or telephone adaptor that we supply, as it can be left largely un-firewalled, whilst leaving your computers protected.
To use a phone behind at NAT firewall, just contact our technical support to explain you how.

What about emergency calls?

Coms.Net is not a telephony replacement service and cannot be used for emergency dialing.