A phone has several levels of software and hardware, the OS that the user knows is not in charge of communications, its main role is to interface the user to the computer inside the phone. The phone OS sees the phone communication hardware akin to the way it sees an Ethernet card. The phone communication hardware (named baseband modem) is also under control of the SIM and every time the mobile operator wants to change the behavior of the baseband modem it can through the Sim toolkit.