Sorry, I must be missing something. I have a couple European prepaid SIMs and none of them used a callback system to make calls. Unless of course I wanted them to.

In some countries I use a callback service from telcan.net (a Canadian company). I dial a number and after one ring I hang up and the system calls me back giving me a ringtone in North America. I dont have to do that but in some instances it is cheaper than a call to the states on a European prepaid. Even if it is about the same it minimizes the frequency with which I have to load the prepaid account. It works great in Europe where the incoming calls are free but I dont think it would help much in the US since the incoming calls arent free. However, you might check their rates to call from the US to Canada, since they may be very cheap. If so you could use your prepaid for US calls and use the callback for calls to Canada.