Be careful doing this I started reading up on this and some info stated that they pay your early termination fee when you trade-in a phone based off how much your phone is worth and if you're early termination fee cost more then your phone is worth you have to pay the difference in your monthly contract. Not sure if this is true or not but it makes sense.
So check to see if this is true, if it is you might make some money selling the phone to pay your etf vs them doing it for you, and you losing the extra $
Now if they pay it regardless of the phone and theres no extra fee then heck yes buy a cheepie phone. Cant imagine this would be the case though.