During registration of your GlobeSpot AntiTheft device, you enter your mobile number.
Only SMS commands send from this registered number, will be answered. 

Would you like to allow more people to have access to your device with their mobile number?
No problem. Via the password secured ADMIN page on the tracking website, you can add more mobile phone numbers.

The number of mobile numbers you van add to grant access to the SMS command system is unlimited and can be removed at any time. You decide.

It's also possible to follow multiple vehicles using 1 mobile number. In this case it's only a matter of having your mobile number registered on the AMDIN page of each device you wish to send SMS commands to.