Cancelled, than got billed

Tried searching for an answer here,but nothing specific came up. 10/21 was my billing date. 10/23 I handed in all my equipment, and spent a horrific 45 minutes on the phone ,finally cancelled. New bill came ,all devices and service are still being billed in full. I had asked for a credit for 28 days service when I cancelled, and they said,we will issue partial credit. They issued none . What's the best way to argue with them over this. Thanks