When I want to make an international call from my iPhone (or from my home phone) I use Skype. There is no native Skype application for the iPhone, so I use Fring,
although today's announcement of a deal with Nokia includes hints that they are working on more mobile platforms. They support Android already, so their main missing link at this point is the iPhone.
Fring is a decent substitute for now. I mostly use it to keep up with Skype and YIM chat sessions when I'm away from my laptop.