VoIP pre-pay phone?


Quite handy for expats because:

- cheaper international calls
- you can access it on your mobile phone, even abroad
- possibly cheaper national calls
- with pre-pay, no contract liability; more secure, no debt

But where can we find such a service in Argentina?

I've found residential subscription plans but not prepay. I can use my UK VoIP service but then calls to BA are 12p/min.