The phone key is based on NFC near field communication that is also used for payment.
You have to register on mercedes me with your phone number. For the starting procedure a mobile phone with nfc as well as secure sim is needed - will be checked by the system.
The service is free of charge for one mobile and three years of use.
Security should be better than the remote key as the distance to the car is much smaller and therefor harder to spy.
As a thief you shoud have the sim data the imei and the protocoll at the same time;)
