The Android app for this is called Extreme Call Blocker, and it's easily the best money I've ever spent on an app. It is event-based behavior that you define. For example, I whitelist my contacts, and anytime I receive a call from any other number, it picks up the phone and immediately hangs up, and I receive no notifications whatsoever. This has the nice effect of denying any access to voicemail. For my contacts, I have ECB let it ring a few times, and then also deny voicemail to people in my whitelist. Obviously I hate voicemail. Then, if ECB ends up picking-up-hanging-up-to-prevent-voicemail on a few of my special contacts after I don't answer (e.g. coworkers or family), I have it set up to helpfully send a "form SMS" to the contact telling them I'm not available and to call again or SMS if it's an emergency. Absolutely the best app I've ever purchased.