Skype beater – free landline calls abroad – how do they do it?

Now this is perfect for ex-pats like me, or for anyone with family living abroad.

The VoipBuster! is a bit like Skype, not as good yet for general use, but it does have one major advantage – free calls to landlines in many countries, including Spain, the UK and the US, once you have paid a minimum 1 Euro subscription.

Seems amazing but it really works – you pay a Euro and talk for free for as long as you want. How do they do this?