Saturday, November 24, 2018

Goodbye phone spam

I've been self-hosting voice calls and SMS for a while now.  Like most Americans, I get a ridiculous number of spam calls.  The main sources of phone spam for me are, in order:

  1. neighbor spoofing
  2. vendors who use my number long after our transaction has completed
  3. calls intended for a former owner of my number
I considered a few options to reduce spam volume.  Since I self-host my phone infrastructure, the following approach was easy to implement and has been really effective:
  • choose two phone numbers in an area code that has never called me before
    • 906 or 308 are good candidates for most people
  • let these numbers simmer for three months
  • give one number only to immediate family ("private" number)
  • give the other number to everyone else ("public" number)
  • block all calls without caller ID
  • block all calls whose caller ID has the same area code as my numbers
  • burn the public number annually
Letting numbers simmer with an intercept message stops a lot of spam in category #3.

Blocking calls from the same area code eliminates category #1 entirely.  Since I use an area code that has never called me legitimately, there are essentially no false positives.  In practice, this also blocks most calls from category #3, so I have belt and suspenders there.

Spam in category #2 is trickier to block since I initially want these vendors to contact me (dentist, mechanic, etc), but want the calls to stop once our working relationship is permanently finished.  Asking nicely works some of the time, but it doesn't help if a vendor sold my number or lost it in a data breach.  Burning my public number annually or biennially eliminates category #2 that can't be handled in another way.

This approach might not work for others, but has proven highly practical for me.

I considered having only a single phone number and using a white list.  Maintaining an accurate white list seemed like too much effort.  I also didn't like the risk of false positives.  For example, if my kids call me from an unexpected number, they should get through immediately.

My private number is essentially a long term token granting access to my attention day or night.  My public number is a short term token granting access during reasonable hours.  For example, when I ask not to be disturbed, my phone infrastructure routes the public number directly to voicemail but still routes the private number to my phone.

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