People need your return number for voicemail (if you decide to opt into that; personally, I don't), and to recognize who's calling them. I know enough people who don't accept a call from a "private number" or "unknown number".

I have my smartphone 24/7 on DND. I only feel/hear my vibration motor if I get a call. Sometimes, this causes me to miss phone calls, but in general it works well enough. It also makes me one of these people who isn't annoying in public transport or on yoga class.