I regularly ignore phone calls, and I tell people - if it's an urgent matter and I don't pick up the phone, call me right back. Two calls in a row means "Pick up, this is important."
For Android there is an fantastic FLOSS app called Harass Me that lets you set an amount of calls in a specific timeframe to make the phone turn up the volume and "harass you" to pick up. I have that setup for emergencies while my phone is silent otherwise.
http://www.rfc1149.net/devel/harassme.html | https://github.com/samueltardieu/harassme