If your dad is the PAH he is basically the owner of the line.

So there’s no why for me to switch from his line to a brand new one

You need his approval to port your line.

Striking out on your own is a really exciting time and being able to take your phone is a big deal. There are some things to know about the process so check out our Transfer account or line ownership page on T-Mobile Support. Your dad would need to call and authorize the line to be moved off of the account in order for you to put it on a new account in your name. You could start a new account with a new number, but that is the only way to start a new T-Mobile account with that same phone number.