Why does my cell phone beep every morning at 3:33a.m.?

For about a year now, my cell phone rings every morning at 3:33 (or a minute nearby). It doesn’t do a normal ring, but makes a single alert beep that would normally mean voicemail received. Yet, there is no voicemail.

Am I haunted by a cell phone ghost or is this a setting I can get rid of? It’s happened on all three of my past phones: LG, Samsung and LG again.