Any app to help save me intl long dist with iphone on other end?

I dont have an android device yet, but will be getting it next week when Epic 4G Touch releases. I am looking for an app to talk to my family and friends, who are overseas, and save some money on international long distance. But they have iphone on their end. Video chat is not a priority, but I want audio to be good. So is there any app that is cross compatible with android and iphone, possibly has a desktop client as well, and can let you do audio/text chatting?? Also, if possible, it should be able to work on carrier's data network, and not just depend on wifi for calling each other. Can you recommend anything?

 

Ok. I researched some more. We all have google voice. Can they use Talkatone on their end and I use Groove IP on my end?? Has anybody had such setup before?