I’ have the watch, and have been trying this out. Unfortunately as of today the answer is no. The watch still uses Wi-Fi to make FaceTime audio calls.
If it is connected to a known Wi-Fi network, the watch will work indepent if the iPhone, but again it will not work on lte/cellluar data :(
Messenger is a no go as well and the “line” app still requires the phone to be nearby as well. I got two of these watches hoping this would be an included feature and now I’m thinking about returning them. I only wear the watch while working out so it’s useless to me ymmv.