It should work only for those phones which came with Avast factory installed. After you do a factory reset and turn on phone, they send an SMS to contact their server. This gives them the phone number which can then be used by the server to send remote commands to the phone.

One of my phones had McAfee pre-installed with a similar setup. If you do a factory reset and they find that it is a different phone number now, the phone locks down (You can enter a pin to unlock). You can't even access the bootloader or do another factory reset.