I assume the recipients don't end up paying as per other mobile calls abroad? I must investigate as to whether one would work here as mobile phones are the biggest rip-off known to man in this country. I assume they need a UK phone number to work?
As I understand it the service is free to both parties. There is a gotgha.
both perties have to be using VoIP. You subscribe to skype for free
http://skype.com/ and you can then make free calls to anyone anywhere in the world. If you want to call a normal land line or mobile then you have to pay a subscription and pay for each call but the rates are very low.
t.