[Sophos]How the "Great SIM Heist" could have been avoided

[igor51]

How the "Great SIM Heist" could have been avoided

Apparently, intelligence services managed to penetrate the network of a major SIM card manufacturer, grab loads of SIM keys, and now we're all liable to be listened in on.

But why? What is it about SIM cards that made this possible?
Source: How the "Great SIM Heist" could have been avoided