I just downloaded this nice application on my HTC Touch Cruise. Once installed, you can make free mobile calls to anyone who has this application installed on their mobile phones. It connects with popular IMs such as Skype, Yahoo Messenger, MSN Messenger, Twitter, etc. I just chatted with my friend in Denmark with fring but I have not tried to make a call to his computer.
This program seems to be able to run on my mobile platforms including the iPhone (still in beta).
Download it, get online and lets test this out.