[GMB]

I have also encountered this handset problem.

I got the impression that the handsets that did this were Racal-type handsets rewired for Clansman (they look like regular telehpone handsets rather than the smaller Clansman kind). I fixed it with a lowish resistor across the microphone.

I had meant to see if blocking up the hollow bit between the mic and speaker would help as the problem did not seem to be feedback through the air.