Thanks andy. Yeah, I've found this good website that shows the providers you wrote:
PrePaidGSM.net: National GSM Operators
The roaming and call tariffs depends a lot on the country, specially in the third world. My idea for example is to have one like that just to use in Brazil. I sometimes travel abroad, but I travel a lot inside Brazil. And, believe you, the pre-paid global sim's tariffs are cheaper than local Brazilian sim's tariffs when roaming here! In Brazil if I get out of my city I have to pay a connection fee plus roaming for each call that I receive or put. And it is expensive if it is a post-paid and very expensive if it is pre-paid!
Although local SIMs can not roam for free in Brazil, the majority of these Global SIMs can roam for free here (and lots of other countries). And to put calls I can use them or maybe trigger any voip callback. The only problem would be that my mobile number is in UK and not in Brazil....but....
....if I could find a Global SIM like that but with its number in US, HK, Singapore, etc. I can use betamax + sipbroker + voxalot to forward brazilian landlines (from sipbroker) to my mobile number for free! So I have now a brazilian number and people will pay cheaper to call it (because is a local landline)! This would be possible because Betamax has some providers that call for free mobiles in US, HK, Singapore, etc
This could be done not only in Brazil, but in every country where the Global SIM has free roaming (more than 50 countries around) and has sipbroker, tpad, free DID or similars.
The problem is: I need to find a Global SIM with its number in US, HK, etc. OR I need to find a very, very cheap provider that can call an UK, Monaco, Liechtenstein, Iceland, etc. mobile number.