My phone has done the same thing. (I have Cricket... carrier doesn't matter)
If you parse the message, You will see it is sending the phone model, phone number, the Android version (6.0.1) and then another thing that looks like a carrier number. This happens to be exactly the same as the version of the Visual Voicemail app that was preloaded on our phones.
It seems that when the phone reboots/restarts the Visual Voicemail app is trying to report something to your carrier. I had not opened the Visual Voicemail app since getting my phone last week, so what I did was open the app, and let it do it's setup routine. Now, when I reboot the phone, I no longer see these backend messages in my Messaging app. They may still be being sent, but I believe they are now being masked from view. I don't see them anymore. Try running the Visual Voicemail app and go through its set up process. It may stop these messages from being seen anymore.