In IEEE Spectrum, July 2007 issue, there is interesting article how some extremely smart hackers pulled off the most audacious cell-network break-in ever. It is still unknown who pulled off the most audacious network hack ever, but here’s (IEEE Spectrum: The Athens Affair) how they did it. This article gives a technical insight.
The illegally wiretapped cellphones in the Athens affair included those of the prime minister, his defense and foreign affairs ministers, top military and law enforcement officials, the Greek EU commissioner, activists, and journalists.
The Wikipedia article https://en.wikipedia.org/wiki/Greek_telephone_tapping_case_2004-2005 contains additional links to press stories and background material.
Ericsson’s Interception Management System user manual (marked confidential) is available on the Web through a Google search: https://www.google.com/search?q=IMS+ericsson+manual or at https://cryptome.org/ericsson-ims.htm.