Phone replaced under manufacture warranty

I bought it through Tmobile, that is why I am trying to dispute the monthly installment payment since I no longer have the phone, T mobile will not help me cancel the monthly payment on a phone I no longer have. And Samsung will not help me either since the deal was done as a trade in. Trust me I've been fighting back and forth and nobody wants to help me. I just can't believe Tmobile will not help me since I have been a loyal cust for the past 10 yrs!!!!