Whitelisting is the best option, I use it for my cable phone (Telekom). Every number is blacklisted (blocked). Nobody gets through who has not been whitelisted (cleared) before. The area code of my hometown and the directly neighbourign districts, the three cellphone network codes we have over here, and a very small number of direct numbers (family, close friends) can reahc me. Nobody else whose landline phone is more than 40 km away. Callcentres have no chance - there are none inside the defined area, and the all operate via landline. However, craftsmen for example usually come from the town I live in, or from the near vicinty of it. They can reach me, so can just any cellphone user.
If need arises, I individually clear a single number from further away.
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If you feel nuts, consult an expert.
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