I changed from T-Mobile to AT&T on July 18th and was still on T-Mobile for an extra 8 days into their billing cycle. They charged me for the whole month.
Called customer support and they said they do not pro-rate charges and it's in their legal notice to charge for the whole cycle. That is a blatant rip off, customers do not know about this and I was a loyal customer for years.
I do not feel telecommunication companies have a right to charge for services that you are no longer receiving. They are out to get as much money out of consumers as they possibly can.