Bring it back program is a joke

I returned a phone a year ago when they send me new phones when I upgraded my plan, but it was never told that I would be getting new phones for all the numbers or I would have not accepted the plan as everyone one on my account already owns theirs outright. Now had one of the lines removed from my account and they are charging me for the device because they didn't record it was returned internally. I sent 2 phones back in the same box and they only recorded one being returned, It has been an absolute cluster F* ck to try to get them to remove the charges. They even admitted to be that the phone they are charging me for  is in use on a completely un related account. But will still not remove the charges and i have been accruing interest for months and ruining my credit as I wait for them to sort out their stuff.  I cant even  remove other lines off the account because of the outstanding balance.