It says for new customers. If you start a new plan will they let you port a TMobile number to the new plan?
These offers are typically for add-a-lines, in which case each line you add to your plan would be eligible.
An existing T-mobile/Sprint number can’t be used as the add a line. You have to port it out (cancelling service on the existing line/account) and then you can port it back to T-mobile as a new line/customer after 90 days.