Being charged for Uverse when it was cancelled

I cancelled my service but am still being charged because the bill was for 1/10/18 to 2/9/18 and I cancelled on 1/15. How can you charge me a full month when I had the service for 5 days? I've called customer service and one person said it would be prorated and one said I have to pay the full amount. Either way, it is now showing past due on my account.