I want to use a regular headset (earphones & mic) with my landline
phone. The headset would plug into the socket on the phone instead
of the curly lead.

I'm in the UK and the cost of ready-made telephone headsets are
about £30 or £40 which is a bit steep for me.

So I guess it should be possible to modify an ordinary headset to
terminate with a 4P4C plug which would then go into the curly lead
socket on the phone. I'm hoping it's almost as simple as:

(a) replace the mic in the original handpiece by the mic in the new
headset

(b) replace the earphone in the original handpiece by the earphone
in the new headset.

What special considerations are there about headphone/mic impedance
or about any additional components which may be needed?

Is there any constructor info or a simple circuit diagram for this?