Thought I'd share this on the very remote chance that someone else is in my same situation.
I had a Verizon branded Galaxy S7, then switched to TMobile about 8 months ago. Was always on prepaid and never had a contract. Being a Verizon branded S7 obviously meant I couldn't get TMo's wifi calling feature. But thankfully the S7 is fairly carrier agnostic if you spend half an hour reading up on how to flash carrier specific ROMs. Long story short I flashed the TMo ROM and turned my phone into a TMo S7 and got all the TMo specific features (including wifi calling).
Given that Mint has a TAC/IMEI list of what devices are compatible, I was a bit skeptical whether wifi calling would work or not. After porting my number over, I was very happy to discover that YES, wifi calling DOES still work on my Verizon branded TMo phone! Hopefully this means that as long as you have an unlocked phone and can flash the TMo specific ROM, you'll get wifi calling regardless of whether the TAC/IMEI is compatible. (for reference my IMEI starts with 353 and isn't anywhere on the list of compatible S7 devices)
tl;dr If you have a carrier-branded phone, as long as it's unlocked and you can flash the TMo ROM to it, you "should" be able to get wifi calling regardless of your TAC/IMEI. Hopefully someone else can verify this independently.
P.S. Not sure if this is a Samsung or S7 specific quirk, but each carrier ROM also controls what phone radios get used. Running a Verizon phone on the TMo network meant spotty connections and not so good LTE speeds. After flashing over to TMo it was a day and night difference. Might be worth flashing your phone to a TMo one if you want the best connectivity.