I stopped AT&T service a few days after month bill cycle. AT&T charged for the full cycle still. when I started service, they took a partial month and a month full in advance ( though marketed as postpaid service) which made the first bill higher. But then anybody expects to have the final bill to the date service was provided and not further. That is not the case. They take for days there is no service till the cycle end. Easy money and didn't expect att brand do these cheap things. Happy I left them.
Whats more, I called service when I got the stmt and the agent also thought my version is correct and said there will be a final bill and to ignore the current one. Well, the next bill came with additional $5 slapped for account maintanence. I called again and this agent says that's how AT&T works. I didnt felt to argue or ask to cancel the maint charge to such a cheap company. The agent was really nice but couldn't help.
Hope this helps you to be informed while stopping business with att and you think you will be charged only till the last day you had service.