Subscriber identity module
Wikipedia
https://en.wikipedia.org/wiki/Subscriber_identity_module
A subscriber identity module or subscriber identification module (SIM) is an integrated circuit that securely stores the international mobile subscriber identity (IMSI) and the related key used to identify and authenticate subscribers on mobile telephony devices (such as mobile phones and computers).
SIM card
From Wikipedia, the free encyclopedia
https://simple.wikipedia.org/wiki/SIM_card
A SIM card is a smart card that is used in mobile phones, to identify the client. SIM stands for subscriber identity module. A SIM card has a microchip, and its use is protected by a PIN. When the phone is powered on, a special number called IMSI is broadcast. The microchip is also needed for some encryption and decryption.
I remember how dead set the government was against strong encryption for cell phones.
They have the SIM card data, they have the firmware, they made sure the flakey baseband processor was never cleaned up...and most of all they made certain that perfect forward secrecy was not implemented.
Still, there is a way to get hard encryption for the audio data... it requires an external Bluetooth headset with built in perfect forward secrecy crypto. To achieve security you need an external device since the phone cannot be trusted. The metadata cannot be protected though.