Directv Collection dispute

I have been sent to collections for non equipment return. I have documentation that all was returned and have spoken to customer service who has repeatedly told me my account was in good standing. However, Diversified collection agency has put a collection out for 351.00. I am a current customer of Directv NOW but will be cancelling that as this process has been going on for over a year and no one at ATT can seem to help me.