I had an interesting experience with ATT. I was into the 6 month of a 2year contract back in 2007. I wasn't using my phone much (non-iphone). But then I went on a road trip to Yellowstone, which included driving through ND, SD, MT and WY. During this 7 day trip, I was using my phone a lot (work + personal). The total amount of minutes I spent in those 7 days were way more than my usual monthly usage (in fact it ate into my rollover minutes as well).
After a few days from returning, I received a letter from ATT telling me that they could not afford me on their network for using so many minutes, as they had to pay their partner n/w in those states (not just major cities in those states). They wanted me to leave their network, in return they would let me have the phone (under contract for free).
I was happy to oblige as I wanted an iphone. I switched to Tmobile for a week and then went back to ATT again (iphone plan).