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My suggestion is to set up a number or short code you call after receiving an unwanted call that blacklists the previously calling number and if enough people note the number as abusive, the telco system blocks it.
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One problem I see with simply crowd sourcing it is the potential for abuse: there needs to be some way to validate that the phones submitting problematic numbers truly received calls/texts from those numbers. Otherwise I can imagine "services" popping up that will submit your enemies' numbers to the list from a bunch of random phones. We're essentially back in the days of email before dkim and all those other technologies that make it such a pain to run your own server without someone immediately flagging you as spam.
You could have the telcos track usage of phone numbers (or preferably something that's harder to easily change) and see how well that coincides with spam flags. If a number/circuit/IP address/other distinct identifier makes a large number of calls which have a noticeably higher percentage of spam flags, it very well could be spam.
For me, Google Voice is perfect. Unknown callers can be screened, known bad numbers get sent straight to voice mail (optionally with their own custom greeting explaining as such), and really bad numbers get completely blocked by Google if you opt in. These really bad numbers are partially the result of people marking calls/texts from those numbers as spam inside the Voice app or online, which is essentially the system that we're currently discussing.
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