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If people were calling him saying his number called them, then his number was randomly chosen as a spoof number from which spam calls were being made

Happened to my wife's cell phone a couple years ago

And it's happened to my skype number a couple times, too

You friend has two options:

• ignore calls, and only respond if he either:
  • recognizes the number, or
  • they leave a voicemail he cares about
• change his landline number (which might only temporarily halt the problems)