will a unlocked sonim xp3 phone work for t-mobile after january 1 2022

I'm on a $15/month prepaid plan (because it's way cheaper than any of the post-pay plans and, since I barely use any data at all, I definitely don't need the maximum amount of the latest number of Gs). The promotion, I'm told, was only available either to those on a post-pay plan, or to those who upgrade to a prepaid plan that was 2-4 times more expensive (maybe both, maybe just the first one, I can't remember). I don't think they were making it up on the spot, either; they had to consult a table that had been printed out from corporate HQ.