iPhone 6s suddenly said Activation Error I picked up my iPhone 6s to use it, and it starting activating itself out of nowhere. Then it asked me to choose a Wi-Fi network. I had no clue what it was doing, so I did what it said, my Wi-Fi network was already checked so then I tapped done.

It then went to a screen that said Activation Error. Beneath that it says 

Activation could not be completed.  Please restore the phone and install the latest version of IOS and attempt activation again.  If the problem persists, please contact customer support.

Whats's going on with my iPhone all of a sudden? I haven't tried the restore route because my phone hasn't been backed-up in a while, and I don't want to lose my Notes and Contacts data.
This is happening to my phone when I get out of range of my cell service. It is really frustrating because I can't take pictures or anything when this happens. But, I have found that if I know that I will be going somewhere where I won't have a signal I put my phone on airplane mode and then it doesn't happen.