Marshmallow, Priority only mode, Viber still rings with sound? I use Android Marshmallow in Nexus 5x. I have an automatic rule for nights where it goes to Do Not Disturb with priority only as set. It works as expected and silences when someone calls me over cell network, but if someone calls in Viber, (an app like WhatsApp) the ring comes in with all it sounds, but I expect it to be silent as well. What I am doing wrong?
There are two possibilities for what's causing Viber calls to ring during Do Not Disturb Priority Mode. Here's how to fix both possible issues:

Priority Only Rules

It could be the fault of your priority only rules. Go to Settings -> Sound -> Do Not Disturb -> Priority Only Allows and set both Calls and Messages to none. This will stop calls and texts from ringing your phone while you're in priority mode (from your stock phone/messages apps and potentially also from Viber).

Override Do Not Disturb

The other possibility is that you're overriding Do Not Disturb restrictions for Viber and letting it sound during priority mode. To turn this off and block Viber calls from sounding during priority mode DND, go to Settings -> Apps -> Viber -> Notifications and turn off Override Do Not Disturb. 



tl;dr: Change your priority mode rules or block Viber from overriding priority mode Do Not Disturb to fix your issue.