Locked phone In contract
So I purchased an iPhone 8 plus on let go and the seller did not let me know that they didn't finish paying the phone off. I tried to put my metro sim card in it and it said "Sim not supported" and I tried to unlock as a non customer on the att website and it said that I have to finish paying the phone off. So my question is if I just go into att and switch my metro number to an att sim card and put it in the 8 plus will I be able to use it? And when I bought the phone the sim card was still in the phone and the phone is still in service. Like it still recieves calls/ text from and it has celluar data.