The GSM-doorbell allows you to remotely open your door/gate/etc. using your mobile phone.
The system is build up of 4 parts:

1. An intercom
2. The GSM-doorbell
3. An electronic lock
4. A simcard for the GSM-doorbell

Ring the doorbell (intercom); the intercom signals the GSM-doorbell. The GSM-doorbell rings the number that is displayed. The occupant answers the call and is able to speak to the visitor. By entering a pincode on their mobile phone the occupant is able to open the door.