Do you need a data plan with an iPhone if the sim card you are using now has unlimited internet?

I am planning to buy and iPhone, but don't know if I need a data plan. I'm gonna use my current sim card that has unlimited internet. I don't wanna get charged extra money just because I don't have a data plan. Anyone have an answer?Do you need a data plan with an iPhone if the sim card you are using now has unlimited internet?
You would need another sim card. A T%26amp;T iphones uses a smaller sim card then the regular A T%26amp;T cell phones. The sizes of the sim card are different. No matter what you would be require to get a data plan. Plus with A T%26amp;T there are no more unlimited plan for the iphone. Only 500MB or 2GB of data internet plan for the iphone.