You start iTaxofon and order it to establish call to some of your friend by taping its name in the address book of your iPhone.

Now iTaxofon determine the friend number, check which provider you set it to use. Then it contact the provider's system (via web or provider-specific API or any other way, depending on provider itself) and place call with desired properties. It also save the call to the calls log (just for your information later), then check how much money you have on your account and how much will be rate per minute on current call.

Please choose your provider from the menu, and you'll more specific info on it.