Thanks Rocketman, but I am not yet able to get it working:
Following the suggestion of this forum I bought what should be
(it may be not
) a standard mobile phone monoaural headset with microphone. It is from Nokia and the 2.5 mm plug is divided into 4 distinct segments by 3 isolators
(this is different from the standard stereo headphone without microphone in which the plug is divided into 3 distinct segments by 2 isolators
).
Well, when I put this Nokia monoaural headset with microphone into the socket a dialog pops up and I have 2 choices:
if I chose headset and then call, hear absolutely nothing and the person on the other side hears my voice very very feeble, so feeble that the first time hung the telephone thinking there was nobody;
if I choose headphone and then call, hear perfectly, but the person on the other side hears nothing.
I hope it is the Nokia headset that is not compatible. Please describe the plug of a working 2.5mm headset

is it a 4 segment-3 isolators or is it a 3 segments-2isolators?