Adding an existing phone number as a second line on an iPhone 18 Pro

I don't know if this is a T-Mobile question or an Apple one. But here goes. My mom passed away over the summer. My dad still has her phone so he is able to get codes to log into certain accounts. He's switching over as many as he can, but doesn't want to get caught without her phone if he needs it. He also wants to trade in his and her 13s and get an 18 Pro. Is he able to take her number and make it a second number on a new 18 Pro where he can get texts to her number? If so, how much would T-Mobile charge per month for that second number? Thanks