Best is when caller and receiver use one service provider, and i agree totally with author it is for free, because provdier did not need to sent data over network to other provider. Voip providers number grow and grow everyday, and posibilities are much better then yesterday.

And about cell phone service providers story is the same.
My provdier offer good deal to call to other network, but minus here is that i need to pay the same sum of cash to call in my provdier network! I think skype is much cheaper to call to anywhere in the world, voip app in Iphone is great and i testing it everyday!

Thanks for info!