Not surprisingly, there are limits. You can't disarm the system with voice commands, and you can't arm it if it's set to require a passcode. You can't change security levels, either. These could be hassles, but it's clear that Nest would rather not risk an intruder getting in by shouting through an open window. This is mainly for arming Secure on your way out instead of a full replacement for your phone or the hub itself.