Trade in an old device on an existing line

Most "Free Phone" deals involve either adding a line or upgrading your plan. I was able to get a trade-in deal that allowed me to keep my grandfathered plan but that was when T-Mobile was more actively trying to get everyone on 5G phones.

The one thing that I will mention is that a month ago I checked and found that the factory unlocked version of my OnePlus 8T 5G was selling for less direct from OnePlus than what I owed T-Mobile on the T-Mobile locked version I bought a year ago. So, you can save a heck of a lot by just buying last year's model direct from the manufacturer.