porting phone number from dad tmobile account to my new account

If I understand correctly, you want to change the phone number of a line on your max 55+ plan but that number currently is tied to a line on your father’s separate account which he intends to cancel?

If that’s the case I would call Tmobile support to see how you can do that. I would guess the first step is to assign a new phone humber to your dad’s line before he cancels thus freeing up the phone number, then just changing your number to the new one that’s now available.

Edit - it seems when you change your phone number, the old number is held for 45-90 days before being able to reassign. That’s probably not going to make my original suggestion possible.