The GSM Gate Opener incorporates a porfessional industrial grade GSM engine that received calls at its SIM number,as any other ordinary GSM phone.The system operates in the GSM module quad  band 850/990/1800/1900 MHZ.Just to give an example,if your SIM has a phone number 92289810,that is the number you have to call to open the gate.The system has a memory where the designated numbers are stored with a maximum of up to 99 numbers.When the Gate Opener receives a call,it will check the calling number and validate it with the listed numbers.If the number is in the list then it will reject the call and open the gate.The system does not answer the call,it simply checks the caller ID.Your network must send the caller ID otherwise the Gate Opener will not work.Usually operators don't charge until you answer the call,so it won't cost you anything to open the gate with the Gate Opener.Of course,an active SIM is needed for this to be possible.To store a number,you simply send an SMS,that starts with a password to the GSM Gate Opener,indicating the phone number to enable its position in the list.Numbers are stored in the system's memory and not on the SIM.This prevents someone from removing the SIM to copy all the numbers into a mobile phone.Only the one who knows the password can insert or delete enabled phone numbers