MerylStreep I remember using your method many years ago before any form of 'monitoring' was available and I was receiving indecent phone calls.
However, I was warned back then that it was possibly a dangerous thing to do as if I damaged anyone's ears I could be sued for damages. At first I laughed but was told the caller may be innocent and just have a stutter or similar impediment which I suppose was fair comment 
Worked for you and me though didn't it 
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