My thought here is that I get that from those robocallers. It appears the robots call numbers looking for anyone answering and saying "Hello?" then it hangs up and marks your number as good for spam calls.
Given your story I don't think this is anything but those robots looking for "Hello?"'s.
I have a randomly occurring problem with my Home Phone. When I answer an incoming call, I get a repeating series of three tones that sound like the "number no longer in service" sounds when a number has been disconnected.. I have to hang up and hope that the caller calls back. Any suggestions on how to stop this? Thank you.

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