Like many others here, I left AT&T but was not prorated on my final bill. I was only 3 days into my billing cycle. I tried to switch earlier but was unable to.
After calling customer service and the office of the president, they never could produce any document where I agreed to the no proration policy. But they still refused to credit me.
I don't care that other carriers do this, too. It's an unfair policy that gouges people for service they don't use. If they can prorate your first bill, they can prorate your last one. I encourage others to file an FTC complaint like I did.
At least I'm now a former customer who isn't getting overcharged for awful service. Thanks Sprint!