Under Settings there is an item called "Notification ringtone." It's not clear what this ringtone relates to. I have a ringtone for calls, another for texts and none for emails. It's none of these. Last night at 4am my San Francisco phone decided to play the "Notification ringtone." I have not been able to figure out what it was trying to notify me of. What is this all about, and can I get rid of it?