This article will help you confirm whether alarms go off when a phone is on silent or do not disturb. On most smartphones, alarms will go off even if the phone is on silent, vibrating, or do not disturb mode. But you should still check the ringer volume and alarm ringtone.

Silent mode does not silence alarms. The alarm will not only sound when you turn off the phone or when the battery runs out. Feature phones can play the alarm even when the phone is turned off, but iOS and Android smartphones do not have this feature yet, because the alarm in modern phones depends on the operating system inside.

Make sure you have the alarm set to a ringtone (something other than 'None') and that your phone volume is set to a level where you can hear the sound.

Here's how to turn the alarm on your iPhone up loud enough so you can hear it.