Public Telephone Cards
An interesting snippet from the Independent newspaper:
Opus Dei financier is found dismembered under bridge
"Carabinieri paramilitary police traced the alleged gang by following the signature of public telephone cards used to make calls during ransom negotiations,.."
European prepaid phone cards generate a unique identification number when first used; this helps prevent cloning of cards, but also enables all calls using the card to be logged. More...
It appears that whenever a call is made the unique card ID and the ID of telephone booth are stored, together with the number called and the time and date.