Call Ended immediately - 1 specific number - only happening with Tmobile

Hola

I will make a long story short. I live in the US and have Tmobile. My mother lives in France and has a French number. For the last few years of her having an iphone, I never had any issues. It's been two days that when I try to call her, it says "call ended" immediately and does not event ring. See below a few key points that makes it impossible for me to find a solution :

1- she is able to call me and it works
2- She receives my texts and I receive hers
3- My dad is at the same French operator and I have no issue with him (she is the only one I have issues with)
4- I tried from 4 different Tmobile numbers (from friends) and it is the same issue for everyone
4 - I tried from a friend at Verizon and it WORKED

What could it be with Tmobile that she cannot get receives calls from the last 2 says from a TMobile number ?