I noticed recently, that when i answer a phone the phone (running stock 7.1.2) the "Do Not Disturb" mode gets triggered. I'm not 100% sure it's coming from the app so i figured i'd mention it. This is the app i'm using to manage DND
Play store: Do Not Disturb (pro) by Cabooze Software
My set times are
Weekday nights from 10pm to 7 am
weekend nights (fri-sat, sat-sun) from 11pm to 8am
It happened just now again when answering a call at 5.15pm which is outsite of the DND frame.