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I am hoping that I am wrong about the single-ring "alert." I sometimes get 10 or 15 junk calls per day, and having each and every one ring my phone a single time would be a continued annoyance. If it's true, then I see no point in implementing Nomorobo and will continue to seek out a call-blocking service or method that really works.
"If a robocall is blocked, then your phone rings once."
Thank you for the info, KevinM2. That's a deal breaker, free though the software may be.