I have used fring successfully a few times, however, there are many times when it just disconnects when I try making that Skype Out call from the iPhone. I am not sure what the issue is here. Connecting to IM (instant messaging) services like MSN, yahoo and Google always works well.
Similarly, I have successfully made quite a few cheap phone calls from the iPhone using Truphone. They have very good customer service: I had bought the American & Canadian Saver package for 1000 minutes of calls to the US and Canada and the Truphone app could not log into my Truphone account from my iPhone for two weeks. When I contacted the Truphone customer services, they immediately helped me resolve the issue and extended my monthly package to reflect on the time I had lost and I went back to making more cheap calls to the US and Canada with my iPhone.
Cheap VoIP phone calls using the iPhone are now only one free visit to the AppStore away!