I absolutely loathe when a phone rings or vibrates during a session - so from the Domme's perspective, make sure it is completely silenced.

Regarding risks, it depends. Would someone go so far as to track your phone to see your location? I've had subs turn their phone off at a "normal" location so if tracked that would show as their last location.

Regarding "do not disturb", on my Android I can set it to ring if someone calls twice within 15 minutes - meaning your phone would ring if she called twice in a row. Don't know if you would want that to happen but if there is a real emergency, at least you would know.