How do I escalate a billing complaint?

In 2019, I cancelled a DTV account and returned the equipment.  In September of 2020, I received my last bill from a collection agency that included a 405.00 charge for unreturned equipment.  When contacted by collection agency, I told them that the equipment had been returned.  They called Direct TV with me on the line and after some conversation and 3 transfers, Direct TV confirmed that they had indeed received the equipment and that they would reduce the payment and credit that amount to my account.  I paid the difference.  I checked on the credit several times and was told that the credit would show up in my October final bill.  Fast forward to March, 2020, I received a call from another collection agency asking for the 405.00 from Direct TV.  I told them that this was not correct and they told me that I needed to contact Direct TV.  I called Direct TV and spent about 2 hours being transferred from person to person who could not seem to help me even though they saw the notes on the account.  I was also told that they saw some credits applied to my account in December of 300.00.  I asked then why was I contacted by a collections agency to pay 405.00.  She told me that she would have her supervisor call me.  Her supervisor called me and after I gave him the information, he transferred me to the collections agency.

After explaining the entire situation to the manager of the collection agency, he then did a three way call with a billing representative from Direct TV.  After some research the DTV representative admitted to seeing the transactions on my account and told me that he was going to refer this account to the "back office" to establish if this information was valid. I kept asking why was I credited 300, but was still being asked to pay 405.00

Then the collections agency manager decided he wanted to make me an offer to pay the bill and then have DTV reimburse me.  I laughed and told him that I was born at night but not last night.  

Something is wrong and fraudulent going on and I need to get in touch with someone who can get to the bottom of this.