My ASUS Zen phone has suddenly started beeping every 15 minutes. How can I stop it?

[AC Question]

The phone is for emergencies and is hardly ever used. It was switched off for 3 months while overseas, now it just will not stop the 15 minute message type beeps. And there are no messages.

 

[B. Diddy]

Welcome to Android Central! Are these the typical notification tones, or are they different?