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

IMO You should consider it a real possibility both that it will continue to work in definitely and that it will stop later in the year. I believe your model will continue to be supported.

But I strongly suggest that you have a Plan B in place for if and when it stops working.

Points worth mentioning maybe:

…Sprint/t- mobile When I checked about a month ago was offering to send me a free Somin XP3 to replace my even older flip phones which they said where going to stop working at the beginning of this year. So it's odd that they're telling you your  XP3 will stop. This is not the first time that I have gotten clearly inconsistent messages regarding this from T-Mobile and Sprint.

…Have you looked into one free phone they're willing to give you to replace your roof which they say will stop working? Why not go ahead and order a free replacement phone from them? Charge its battery, and stick it in a drawer. If the present phone you have keeps working leave it in the drawer. If your present phone stops working Take the new free phone out of the drawer and activate it

…As of today I have two Sorint  flip phones (One really ancient Kyocera, one Dura XT) no I was told would go dead at the beginning of this year. Both of them are working fine at this moment. I have a Sonim XP3 on a separate t-Mobil account, It is also working.

….I note with surprise that my older Sprint phones …assumably communicating with the old sprint towers…I actually have areas of reception on trails in a hike with the phone with the T-Mobile account, even though it is a newer  phone, has no reception.🙄

does that make sense?

Al