Sipgate - number is free, allocated when you sign up
you can receive calls / get voice mail recorded & listen to it without putting any credit on it
Draytel - number gets allocated after you put £10 credit on. Until then you can try it with the 10p trial credit that you get, or you can put £1 credit on via a premium rate call to make a few calls.
Once you have put £20 credit on, draytel calls are cheaper than sipgate for me
The great thing about them is that when your equipment is switched off, nobody can call you. I use one for working from home or when they are cheaper than bt (most of the time unless there is talk time in your bundle) and one for DCAs
There are some voip providers that give free calls to some countries once you have put some credit on, so it's worth looking around, depending what you want to do & how frequently.
I use an adapter between my router & an ordinary phone, so calls normally go via voip, but incoming calls via bt or voip go to the same phone (but not both at the same time)
You can have more than 1 supplier (I have 2 profiles to use in my adapter & 2 different suppliers)
Be careful if you look at vonage - you might end up with a years subscription
Also be careful about any supplier who provides free or cheap adapter (if you want to use one) - it might be tied to a particular supplier, whilst an adapter is fairly cheap & easy to configure & if you look around you can get one that suits your requirements.
Grumpy
voip phone services 
