if you have a fixed phone, or at least a O2 home zone number, and you can tolerate a lot of silly commercials :D, I would recommend www.peterzahlt.de.
you can call to China and Germany and some other countries (fix phone) for free.
Two disadvantages:
1. the call will be cut off every 10 minutes and you have to dial again. One way to extent it up to 30 minutes is to pay attention to the commercials and answer the questions coming up.
2. sometimes you have to queue before you get the connection. And on Sunday afternoons it is especially bad. But you can solve this problem if you have a fix phone flatrate, then you can choose the "Selbsteinwahl".