IF you have a WM phone and use the internet, BUT don't close the application and is left on a website that refreshes every so often you will incur data.  You have to go to the task manager and physically turn it off.  if you don't it'll refresh the page.  Not sure if the matrix has something similar.  I know the Matrix Pro, and the DUO (which i used to have) did that.

Someone has already mentioned this.   i should read better.  oh well.  good luck to you guys, and I hope you get everything sorted out.