If the timer alarm doesn’t ring

Due to limitations in iOS, if your phone is in Silent/Do Not Disturb mode and the Timer app is terminated, no alarm will be played, however you will receive a notification. Don't explicitly terminate the Timer app while it's in the background.

NOTE: The app will play the audible timer alarm even if the app is running in the background and the phone is in Silent mode.

I didn't terminate the Timer app, but the alarm still didn't ring.

There are a number of circumstances where iOS will terminate an app while it's in the background:

To be safe, turn off Silent/DnD mode when you have an active timer.

Will the timer alarm ring if I'm on a phone call?

No, the timer alarm sound will not play while you're on a phone call, however the alarm will play once you end the call.

Can I stop the timer alarm without unlocking my device?

Unfortunately you cannot silence the alarm directly from the lock screen. This is a limitation with iOS and not with the app.

However, if you tap the notification and unlock your device, the Timer app will launch immediately.

Adjust Alarm Volume

You can adjust the alarm volume in the Timer menu. Tap the menu button in the top right and increase the "Alarm Volume" slider to make the alarm louder.