The A5/1 encryption algorithm used in (traditional) GSM networks were proven to be breakable by brute force back in 2009/2010. This means that GSM calls can be intercepted, decoded and listened to by anyone. (SMS also, but that is a different story)
To do this easily you need a 1600 GB big rainbow table.
These files are/were available on bittorrent; a mirror of the torrents is available. The original source on reflextor.com is unavailable.
I’m offering the files
for sale on disk or as paid download.
The intent with this is to facilitate further research and experimentation. Drop me an email and we’ll figure out the details.
Updated 2013-10-23: Clarified language.
Update 2016-05-10: Too much hassle shipping disk drives from outside the EU. From this point on I’m offering access to the files on FTP only.