Q: Being charged for data; no data plan

The title says it all. I have an iPhone 4, and I'm being charged for data even though I don't have a data plan. I only use wifi to access the internet. If I don't have wifi, no internet access. I got a message from my phone carrier that I now have a bill of $217.32! I don't even text or call that often, I also have an unlimited texting plan. I have been taking out my SIM card recently to avoid "using data". My parents have the same plan as I do, but they use Android, They don't get charged as much as I do. Anyone know what to do?