how to get a number for a person who has t mobile cell phoner?

Answer 1

Ask them for it. If they don't tell you then they don't want you to have it (that's their right to privacy). I'm guessing that you knew that already. If someone doesn't want you to have their cell phone number, then why exactly would you want it?
I'm hoping that I misunderstood your question and you mean that someone has a t-mobile phone with no number assigned to it or no account for it. Then you would just contact t-mobile (online or store) and get an account for the phone.