The most frequent kind of calls are the calls with single intent/question.
Knock fixes this with responsive missed calls, which shows a message in the incoming call screen and gives you quick response options.
This is how it works:
You send your friend a message ("Where are you?", "Is the movie plan on?", "Did Brazil win the game?")
He gets an (simulated) incoming call with the message overlaid on the screen.
Instead of answer/reject he will have options "Yes/No/Share location".
He can with one swipe respond to your question.
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