"Anyone can record DTMF tones in a voicemail message and forward calls to that number."

I have never used "duo" and it has taken me a few reads of this to understand exactly what this is, but I think it's worth pointing out that your own personal 'dontduo' service would be trivially simple to set up in a simple twiml bin, at twilio.

I think it would look something like this:

  <?xml version="1.0" encoding="UTF-8"?>
  <Response>
  <play digits="1w2w3w4"></play>
  <Hangup />
  </Response>

"Include w to introduce a 0.5s pause between DTMF tones. For example, 1w2 will tell Twilio to pause 0.5s before playing DTMF tone 2. To include 1s of pause, simply add ww."

https://www.twilio.com/docs/voice/twiml/play#attributes-digi...