My iPhone has suddenly started receiving garbled text messages. It isn't all the time, just occasionally.

I'll get an alert like I've received a text message, but the screen will be completely grey, there are a few lines of random symbols, and a white button down the button that says "Dismiss."

Once I hit the button the screen goes away and everything goes back to normal, but if I go to messages there are no new ones there. I've got no way to tell who sent me the message or what it said.

I haven't downloaded any new apps or changed the settings recently. Has this happened to anyone else? Anyone know what to do to fix it?