Sprint had/has a similar offer to bring your Verizon phone over (with free service + paid taxes and fees for year!) as with this offer it had to be newer iPhones and specific old iPhones. With Verizon its a bit easier because for the longest time they were not locking their phones per some merger agreements, but these days they are locking their phones. FYI: the offer was to expire end of July but as of September they keep pushing forward date. It was a great offer with taxes and fees in the $3-20 range depending on the jurisdiction.

In the Sprint offer they were very clear providing a model list of what could be brought over, and it seems like T-Mobile also disclosed the compatible phones fairly well, at least it was in the red area for a bit so it was readable.