I stumbled upon this ROM on XDA called Jaguar OS. I never heard of it but the pitch contains this:
Another unique feature is protecting the phone from over-the-air attacks. We block responses to Type Zero SMS. Type Zero (not to be confused with Class 0 SMS) is a stealth message sent by an adversary to your phone to determine if it is online - the first step preceding the attack. Type Zero cannot be seen by a user, does not appear on screen and does not get saved. Instead, your regular Android silently acknowledges receipt. As a result, the sender gets a 'phone is online' response. Jaguar does not acknowledge Type Zero SMS resulting in a sender receiving a 'phone is offline' reply.
I couldn't really find much info on these Type zero SMS. Are they actually an attack vector? Has this been addressed in GOS?