Phone number for contact is wrong and won't let me change it

A couple of months ago, I added my partner to my T-mobile phone plan.  He had a straight talk plan. When he switched over to Tmobile, he got a new Tmobile phone, as did I.  They assigned him a new phone number originally and told him he could port in his old phone number once everything was turned on and set up. This was quite an ordeal but they did finally switch everything over to his "old" number. My problem is this:

On my phone - the only way to text him is to text to the "new" number - even though it has been given to someone else now and if I SMS to the "new number" it goes to this other person. But if I just text regularly, it goes to his phone which is set up with his "old number".  If I text his "old number" he gets the text but when he replies to me - it comes through my phone on the "new number." If I call the "new number" I get this other person.  I have tried deleting his contact information and adding him back again but it still does this. It is driving me nuts. I have accidentally sent SMS to this other person as well. Any idea how to fix this? I have called Tmobile a few times and they haven't been able to fix it.

We both have a Pixel 6 phone, although I don’t think that has anything to do with it.