A SIM card stands for subscriber identification module (SIM). This is small integrated circuit board that is unique to each individual mobile phone user. The SIM card is a place to store the key identifies that person's phone number and all their information about what's on their phone. March 2013, a T Mobile SIM card costs $6.99 if purchased in the store. If the SIM card is purchased at the online store, the company gives you a $6.99 discount, making the SIM card free.