When a friend of mine first started using Facetime, he had this problem when he called his daughter's iphone 4. At first, he was using the e-mail address associated with her iphone. However, the problem was cured when I suggested that he start using the actual phone number for her iphone 4 to contact her. He was under the impression that if he used the phone number instead of the e-mail address, that the Facetime call would go over the cellular network, and be charged for it. It still uses Wifi/internet.
I'm just throwing this out there for you as a possible solution.