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By having the phone off hook I carefully inserted my little screwdriver here and there until a heard a click in the handset. And when I found that click I injected a signal from my MP3 player – and it worked! Then reverse, by blowing in the handsets microphone I used the regular headphone for my MP3 player to see where I could ‘hear’ the phone. By grounding on wire and using the other as a probe I quickly found the spot next to the first spot.
Not for the faint of screwdriver.