are calls between iphone to iphone free?

FaceTime is free if each user is using WiFi. If you're on the road without WiFi then it is not free and uses data. FaceTime itself is free and works across iPhones, iPads and Mac computers. Any of the following can call each other via FaceTime iPhone, iPad, or Mac computer.

Calls between cellular devices use carrier minutes. Many offer free mobile to mobile, however that is up to the carrier. One advantage to using the iPhone is the ability to make Facetime calls if you are connected to wifi. Many carriers now are offering Facetime over cellular network if you are using iOS6. Using this however, uses your data. There are also apps like Skype that you can use over wifi as well, but to truly answer your question, calls between iPhones are not free.

To clarify, the only cost involved is your normal monthly fee for your cellular data usage (or any overages you may incur) with your carrier. Facetime is free to use, but you're responsible for the required internet connection, whether wi-fi or cellular.

My Facetime is just as 'free' using my cellular data as it is using my personal home wi-fi.

Check out Viber in the App Store.

Depends on your carrier, example, verizon to verizon yes, but by default iphone to iphone is not. Its a carrier determination.