Re: Random text sent- What is this?
I believe I figured this out. My AT&T Galaxy S7 was doing the same thing, which is how I found this thread.
If you parse the message, You will see it is sending the phone model, the Android version (6.0.1) and then another thing that looks like a version number. This happens to be exactly the same as the version of AT&T's 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 AT&T. 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.