According to leaked documents from former NSA contractor Edward Snowden, the NSA and the GCHQ managed to decrypt cellular communications data by stealing SIM card encryption keys from a manufacturer. The two agencies formed the Mobile Handset Exploitation Team (MHET) for this purpose back in 2010.
The American National Security Agency (NSA), and the British Government Communications Headquarters (GCHQ), similar clandestine intelligence agencies, stole SIM card encryption keys from a manufacturer, allowing the groups to decrypt global cellular communications data. The effort, according to The Intercept, took place in 2010, with the agencies breaking into a Dutch firm, Gemalto. The hack provides the government agencies the capability to monitor “a large portion of the world’s cellular communications,” according to the report. We are of course supposing that the government’s descriptions of its own capabilities are accurate.