I think all current and potential customers should be prepared for the way you'll likely be treated. Never pay a bill late, stay a customer for decades, then cancel your service. THEY DO NOT GIVE REFUNDS even if you pay for the whole month in advance and cancel before the billing period is over. In my opinion, this is just greed and bad customer service.
I also found out directly from an AT&T rep that if I had owed anything when I cancelled, they would have turned my debt over to collections THE VERY SAME DAY I cancelled.
You decide. I'm glad I dropped them, and they showed their true colors just when I was considering signing up for U-verse by mistake! I'll never buy another AT&T product or service as long as I live and I won't hesitate to inform everyone I know how greedy they are.