You may very well have read about the latest leak supposedly sourced from the secret data stolen by whistleblower Edward Snowden.
The three-bullet version tells approximately this story:
• Intelligence services managed to penetrate the network of a major SIM card manufacturer.
• They got hold of large amounts of cryptographic key material.
• They can therefore eavesdrop millions, perhaps even billions, of mobile devices.
Having copies of the keys in the story doesn't just let you listen in to present and future calls, but theoretically to decrypt old calls, too.
Understandably, a lot of coverage of what The Intercept has boldly entitled "The Great SIM Heist" is focusing on issues such as the audacity of the intelligence services.
There's also speculation about the possible financial cost to the SIM manufacturer connected with (though not implicated in) the breach.