Hi, I have a new Iphone4 and to buy it I've done a minutes plan for two years.

My problem is when I spend to much time in the phone in one month. I would like to have an App to control my costs. What I look for is an app the blocks me from doing more calls after I've finished all my minutes for this month. Something like a parents control for phone time...
I've been looking the web for it and didn't found anything.
Does someone knows some app that does the work??

Thanks you for your help everyone!