Apple continues to bicker over allowing and dis-allowing VoIP calls on the iPhone, but one software has come up with a free app that lets you do that without any problems. Its called Pennytel and I speak from personal experience when I say that it works flawlessly.
Of course it requires you to be within WiFi range for making calls (failing which you could use the call back feature / smart dial which could depending on your plan still work out to be cheaper than dialling internationally. Most plans come with about 50 free texts too.
Pennytel has plans that charge you a bare minimum of 5$ a month and their rates are amongst the lowest on the net too. Its an Aussie company , but that doesnt mean that those around the world cant take advantage of their low rates. The best plan is of course the one with no commitments and only 8 cents per call - untimed! Check out their plans here.
I think iPhone users should visit Pennytel and have a look at the User Guide. Its one of the must have applications for international callers. Pennytel doesnt require jail breaking , its easy to download like any other app from the app store.