with Nokia's 3510i there's a headset HDC--5 which lets you use
microphone and headphones for a conversation. The headset connects to
the phone with a 4-pole jack plug.

It must be possible to use the jack plug connection to record
conversations from the 3510i, but I don't know how to wire up the
connection, which wire is ground, which is microphone. which is
speaker. But there's nothing in the catalog at the back of the Nokia
manual to do such recording, and when I rang up Nokia on their 75p/min
number they couldn't answer my question - I was told to ask a small
mobile phone repair shop.

Has someone got an answer for this? Either a ready--built lead to plug
into a tape recorder, or the pinout of the 4-pole jack plug so I can
wire a lead up to do the job.