A:Answer That phone doesn't exist.
There are trade in offers for the S21 Ultra though, yes.
T-Mobile usually offers higher trade in value than Best Buy when you choose to pay the phone in full and T-Mobile Help on Twitter will unlock your phone as soon as you've used your phone on your account for a couple days when you provide them with proof of purchase, so trading in your phone with T-Mobile through Best Buy is really nice if you want to unlock your phone quickly and still receive a higher promotional trade in value.