If all your calls are in the UK and to landlines you might be better using click here
click here or click here
You do need to register - free- and use a credit card for payment. All geographic numbers are 5p connection only and you can talk for as long as you like - no sudden charges after an hour.
The problem with VOIP is that, unless the person you are calling is using the same VOIP provider, there is normally a per minute charge.
Which is why using one of the carrier pre select (CPS) providers above can be cheaper.
If you want a VOIP adapter the cheapest I have seen is click here I have one for calling friends who have Skype and it works very well.