There is a neat feature on the iphone to silence calls from numbers that are not on your contact list. I do this. Not the best solutions, like if you are looking for a job and waiting on a call, or a doctors office where they dont call back on the main line, if it is important, they will leave a voice mail.

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95% of my silenced calls dont leave a message, I assume they are spam "car warranty" or "I want to buy your house!" type crap.

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Do not "decline" the call on your phone or watch. I believe this hangs up on the call, which in turn signals back to the caller that it is a valid number. They use auto-dialers that just call 603-555-1111, 603-555-1112, 603-555-1113 etc until someone answers, then the call is routed to a phone rep who reads the script (or a recording is played). If it is a valid number, they will keep calling.

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(I started using the silence unknown numbers feature during the election when my phone was blowing up)