91av

Liberalisation of telephone networks around the world has made it easier for fraudsters to get free calls from the type of payphones you now find in pubs and bars, says Landis & Gyr of Switzerland (WO 00/70850). As phone companies no longer control which phones are payphones, they can’t tell if someone has found a way to bypass the pay mechanism. The fix is a hidden surveillance box that connects across the phone line where it enters the building. When first fitted, the box measures all the electrical parameters of the line, such as its resistance and capacitance, and the phones’ dial tone and ring characteristics. This makes up an electrical “signature” that is stored securely in the box and checked every time a call is made. Any mismatch warns of potential fraud.