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

Glad you got that clarified.
Sadly, periods it is not that unusual to get inconsistent or in accurate information from cellular carriers customer service it can be very frustrating.
(IIRR I think I had mentioned I believed your XP3 phone would continue to work. I have the same model.)

Re "the promotion for the free replacement is only available to people paying for a much more expensive plan that I have”:

Humm.

That’s odd that you say that.

I have two other sprint phones that are in ancient plans cheaper than any plan you've ever heard of. In fact so low cost that if I told you you might think I'm BSing. The  point I'm making is that Sorint/t-mobile IS supplying me free phones for those two, as those accounts are using phones that will not work after the end of May