Work–Rest Loop Timer
Set a timer and a rest. It runs the timer, then the rest, then starts over, again and again until you stop it.
Works with the phone locked. The beeps play as media, like a song, so they keep going when the screen is off. Turn up your media volume. You'll see a playback notification, and you can pause from there. It beeps only when each part ends and the next one starts.
1:00
Round 1
Paused