About a year or two there were a ton of companies that offered you free calls if you listened to an advertisement. It seems that most of these services either disappeared, or are not doing what they can to advertise. I just came across http://www.freephone2phone.com/index.php which will allow you to make unlimited free 10 minute ad supported telephone calls to international destinations. It appears that it will only permit you to make a 5 minute call to the same number if you make multiple calls in the same day, but I have not seen any other limitations to the service.
I am going to have to ask my girlfriend to give me her actual telephone number next time we talk, so that I do not need to deal with the constant connection problems when using cell phone apps to keep in touch while she is back home in Thailand.
Try it out, then leave a comment for everyone else here about your experience, or if you know of a different service that allows free international calling post a comment for other readers to try out!