"Intelligence officials discovered that when they switched off the tampered phones, two lines would disappear from the network, and when switched on again, two lines would reappear, even though only one SIM card was actually installed in the phone.
The purpose of 'twinning' is to allow third parties to remotely access the data records of the phone, trace its location and eavesdrop on conversations in the vicinity of the phone, regardless of whether the phone is switched on or off.
'The benefits would allow you to eavesdrop on the phone communications,' said Taher. "'f you can also activate the hands-free, you can listen in on what is going on in the room, even when there is no phone call being placed on the phone, so it's an open mike on your target the whole time.'"
Holy crap this is a scary article if you like to have phone privacy. Essentially if some nefarious person gains inside access to a cell phone system, your privacy is screwed. They may even be able to use your phone as a microphone to listen in on you whenever they like.
The joys of technology have unfortunately been followed by the fears of spying. Like the Internet, apparently our cell phone system has not been well designed to prevent infiltration. What a bummer.