I saw the link on Adsense here on the forum and I found it very interesting:
http://www.rebtel.com
I tryed a test call to my CallKeyOne number here in Italy, calling from landline (I have free national calls) and also from my mobile to Travelsim, but I forgot I have no credit on TS so, of course, it didn't work.
The call from landline (Wind) to KallKeyOne using Rebtel was not bad, there's a little delay and quality is not perfect, but it's incredibly cheap.
The interesting (and a bit complicated) aspect is the billing and the way of calling.
You pay 1US$ per week (only if you use the service) and you can do unlimited 30 seconds calls for free. During the 30 seconds you can give your "friend" a local number of his/her country where to call you to, paying just a national call fee of his/her provider. Then he/she hangs off and calls the local number while you keep the line on and you'll be connected free of charge.
It's a bit complicated and I hope I understood it correctly... I hope to try it in November when I'm going to Malta, and maybe even sooner if I'll go to Slovakia and Poland. Of course it's good with local prepaid simcards, not much with international sims.