Complaint Review: Walmart family mobile

 I cancelled my account in October and ported my number over to a new carrier. Called customer service directly after getting the new phone to make sure they knew I was no longer a customer. Paid my last bill that came in the mail and figured I wouldn't have to pay anything else. Today, I receive a letter saying I owe 43.29 and they are handing me over to the bill collectors. Is this some sort of sick joke? I haven't even used the phone and I have the paperwork from when I switched over and the bank statement of when I paid the bill. I refuse to pay them anymore money when their customer service is a joke and can't even figure out what is going on.