I'm not pretending to fully understand how this works, but a new app called TrueCaller is now available which will help you match unknown numbers with the person calling. Operating in 20 countries (sadly, not the UK) including Italy, Spain, Sweden, Germany and Cyprus it will show the real identity of those random numbers when a call comes in.
You can watch a video to see it in action or get it direct from get.truecaller.com. It appears to be completely free.
I'm not pretending to fully understand how this works, but a new app called 
(1)
