Hi Nick.
Thank you for your very quick reply.
It does indeed have a carbon mic (which rustles and crackles as you move the handset) but I will indeed try shorting it out.
I've just swapped over the dial from the 741 - it's a different kind (labelled "54A/EMT 83/2" in case that means anything to anyone) but it works fine and all the nasty "clacking" noises are also much reduced in the handset - just a soft purring as the number is being dialled.
I'm not with BT - I'm using a Grandstream HT701 VoIP analogue adapter - which normally works fine with these old phones as long as the mark/space of the dialler is slightly altered by slightly bending the fixed part of the leaf switch.
Thanks again Nick. I'll try shorting the mic and see what happens.
Kind regards.
From Mike.