Dear Auntie TUAW: Did I overstep my data limits?

TUAW, I need your help please. I am in Mexico and have an iPhone with a 150 MB data plan and 300 minutes. My carrier says that I have passed my data limit and owe them around $150 usd. According to the Cellular Network Data on my iPhone, I have used 35MB and 83 minutes (Settings > General > Usage). My question is, is this information technically good enough to fight back these charges? I think it must be, but I have to be sure. Can you help me, please?