Telemarketers love to harvest phone numbers from the Swedish company registry. Our contact details there used to include Per's mobile phone number, which over time became a threat to his sanity. When he wanted a break from this and threatened to give them my number instead, I took this as an excuse to hack together a simple voicemail service using the 46elks telephony API. It plays a pre-recorded message, records whatever the caller says, uploads the resulting file to S3 and posts a comment in our #phone channel on Slack.
Since telemarketers never leave a message, it's a pretty effective filter.
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Source code on GitHub: https://github.com/kollegorna/telefonista