AT&T never got my returned phone. What should I do?

In February, I sent my phone in for the return program but AT&T apparently never got it. I called back in March and they said they’d look for it. I figured because I wasn't charged the $275 for the returned phone that everything was fine. Now I look and see that my bill includes the $275. I really don’t want to pay that much, especially because I sent the phone in and I have a picture of the box too. What should I do? (I did call again recently but all I’ve gotten was another case put onto my account; never got the case ID either).