The SIM card is used if you don't want to pair your phone or don't have one. With a SIM card in the box, it acts as stand alone phone. You will need a SIM card from a provider, and of course pay the service.I'm really not sure. I was reading the instruction manual before it arrives and it mentions a SIM card. I didn't know if functionality was reduced if you paired your phone to the box vs having a SIM card.