Buy the right SIM card to transfer account from S4 to S6

hermhart

    I use my Galaxy S4 as one of the lines on my son's TMobile One plan.  I just bought a new locked TMobile Galaxy S6.  I want to move my Tmobile account and data from my old S4 to the new S6.  I want to buy a SIM card on Ebay.  How do I be sure to get the correct one? 

    I know I should NOT get a prepaid one.

    Should it be activated or unactivated?

    My phone is locked.  What does that indicate about which SIM card to buy?

      All replies