The phone number is tied to the sim so switch out the sim and go to your
MyTmobile account and change the name associated with the number to who you
want to have the number.
Unfortunately, that won't work for me because my intent is to make L1 free and L9 paid (due to some reasons probably not relevant).
Hey 9xs6vn! I'm sorry to say that in most cases there's not a way to alter which mobile number a promotional bill credit is assigned to, so if those types of credits are what are making lines 9 and 10 free in your case, making changes could very likely result in the loss of the promotion. To be 100% sure, I would encourage you to Contact Us and check with our Care team or T-Force (our social media folks on Twitter and Facebook) so that they can take a look at the specifics of your account! If there's a way to do it we definitely want to help - but I don't want to oversell this possibility, either.