Each line has its own contract. If your line was a 2 year contract, there will be a penalty for switching to Verizon if you port your phone number from T-Mobile to Verizon. You could hypothetically keep the T-Mobile line active and have a Verizon phone too with a different number, or your parents can get Verizon and then switch you over after your contract is up. Call up T-Mobile to find out when the end of your contract is. If you change the plan on T-Mobile, they'll probably extend the contract though, so you might be just as well off by terminating early.