I think 18866 are the same people who own/run
www.voipcheap.co.uk/en/index.html which is voip but doesn't give you a phone number and has ****e support for technical issues.
vonage and freetalk etc are attractive as they are mass market toys aimed at passing trade and non techy people - they also allow you to connect your normal phone to it so it has the appeal of being an (almost) replacement for the BT line.
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the ideal solution would be something like sipgate for incoming calls (so you have a normal phone number) and voipcheap or 18866 etc for your outgoing calls.
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the £80 for a year upfront is a bloody good deal though - if it works! as it includes hardware which would cost you at least £20. my voip box cost me in the region of £50 to get up and running.