i use brastel. it's great. there's two modes, one is high quality, the other low. the low quality isn't bad, just sounds like a cell phone or something. and it's really cheap. you can charge the brastel card at family mart, lawsons and maybe some other konbini
I use Brastel with my work cell phone. Calling to the US and the rates are good. Only thing I don't like about it is having to dial all the access numbers, but I guess that is comman with all phone cards.