Just spent over an hour trying to find out why my ATT bill went up $113. I finnaly got a CSR who could answer the question, which shouldn't have been that difficult in the first place.
It seems when you make a change to your service, I increased my internet speed since they had a faster one I could use, they stopped the bill and then started a new one. The problem is that they come up with a new billing date which is in the future and then they add a prorated charge to cover the time between the old billing cycle and the new one.
It would be nice if that was stated on the bill and even nicer if all CSRs knew that and added that info as a comment on the bill.
Reminds me of a opposite situation. I sold my car because I moved to a city, downgraded my insurance to cover rentals only. My latest bill got prorated for unused time, and I ended up with a 100 dollar credit on my account. Didn't get a insurance bill for like 6 months while that was being used.