I use skype for all of my international calling, and occasionally for domestic calls in Japan. Their rates are really cheap to free depending on how you use the service.Worldlink may also come in handy for those times when you are not near a computer or wifi spot:
http://www.worldlinkjapan.jp/e-index.html
Although, an iphone running skype, and using im+ for skype has supplanted my need for that service.