I use Voipcheap.com, and it does what it says on the tin, place 10 euro into account adding VAT makes it more, hence the 11.70 ish figure that is charged, after that calls to the countries that are on the home page as being free are really free, no connection fee, there is a limit to 300 minutes a week, after that the calls become chargeable, at rates that are shown on their call rates page.
The 10 euros gives you these 300 minutes a week to the free destinations, for 90 days, however you don not loose the 10 euro, it would just mean that if you didn't top up your account after 90 days, the calls would no longer be free.
I use my 10 euros on calls to mobiles and to 0845 and 0870 numbers, as it also helps keep the home phone bill down.
It works a bit like a mobile phone, place credit, get free minutes, and you get to keep your credit to spend on non-free calls, with the limit being 90 days between credit needing to be placed to continue getting free calls.
I did try Voip Buster Pro, this was 365 freedays, for the 10 euro with a bonus 5 euro of calling credit, however i didn't think this was all that great, as they took the 10 euros as a charge for the freedays, and also started to charge 3.9c connection fee, that they were taking from the 5 euro free credit, so ineffect, this way, you end up paying twice for the call, at least with the Voipcheap option, you don't get charged for the free calls, nor is their a connection fee, and you are using the money on calls that aren't free, using the Voipbuster service, they took the 10 euro for the free calls, and also charged a connection fee.
So, i got stung with the Voipbuster Pro, i went back to using VoipCheap.com, both these are run by the same company, and i also think that their websites can be very confusing, if i knew that Voipbuster would take the 10 euro and also charge a connection fee, then they would never of got my cash, and this was after using a sister service, that i do think gives much better value for money, the annoying thing is both sites have nearly identical information in the FAQ sections!