Step 1: Get Another SIM Card
Last Friday I went down to my local T-Mobile shop and had my personal cellular plan converted to a family plan. Then I had them issue me a second SIM card, adding a new line to the plan. I told them that I’d use my old phone, an old Motorola Razr v3. I even brought it with me to the store. We used it to confirm that the SIM worked as expected.