This would be so much more practical than keeping that cyanide capsule in your tooth.
We'd quickly end up abusing this. It wouldn't be long before we just walked around town with our phone ringing at all times, ready to pick it up the moment we see someone we don't want to speak to. Or, even better, we could ignore the ringing to let desirable people think they're important. What we really need is a "Fake Real-Life Emergency" feature to help us get out of unwanted phone calls. If could just project the sound of gunshots and ambulances in the background so we can say, "Oh God there's bullets everywhere!" And then hang up with the caller being none the wiser. Build it, Nokia!