NSA and GCHQ formed Mobile Handset Exploitation Team (MHET) in April 2010. Unit was built to target vulnerabilities in cellphone networks. Operation dubbed DAPINO GAMMA, the unit hacked into a Digital security company Gemalto, the largest SIM card manufacturer in the world, and stole SIM Card Encryption Keys that are used to protect the privacy of cellphone communications. Once stolen, it grants the agencies the ability to secretly monitor “a large portion of the world’s cellular communications” from 450 wireless network providers without the approval of telecom companies or foreign governments.
Source: https://thehackernews.com/2015/02/nsa-hacks-sim-encryption-keys.html