ATT has a plan that is meant to screw it's customers, only offering either a 400 minute plan (which we all go over) and a 900 plan (which most don't use). They also have rollover minutes (unused minutes rollover to the next month and are added) that are good till the end of the year.
I've been able to save a lot of money by keeping a 900 minute plan on my iphone for the first half of the year, racking up a TON of rollover minutes, and then dropping my plan to 400 minutes for the last 6 months. With my average usage, it averages to the equivelent of paying for a 600-700 minute plan. Saves me roughly 30 bucks a month!