Anyone else get the ole' bait and switch?

I got an offer on an upgrade. I was told to turn in my current phone and get $500 promo on the new phone.  I followed all instructions and sent my old phone back, got my new phone and I thought that was the end.  T Mobile is now telling me I have to pay off the old phone.  At no time was this disclosed to me in any way.  It would be as if I went to a car dealership, traded my car in, bought a new car, LEFT then was told I had to pay for the car I traded. Why would I pay for a phone I don't have anymore?  This is bait and switch and deceitful on the part of T Mobile.