GSM Wireless Communicator - Single Station Intercom with Front Keypad- 12V

Requires 12V power source at the gate/door
1. Someone turns up at the gate
2. They press the button on the intercom 
3. The unit rings one of 3 numbers you enter in the unit
4. The first number does not answer the call it automatically redirect to the next number and if that does not answer then to the 3rd number
5. You answer the call and maybe your in town and it's a delivery you talk to the delivery person and tell them to leave your parcel near the door, 
you press the # on your phone and the gate opens to let them in.

This unit operates on 12V - there is a Solar version available also. 

Each GSM requires a Sim Card, The best value I have found is from Kogan.com.au
Network - Vodafone
Unlimited Calls Aust-wide for 12 Months for $179.90 = $3.46 per week.