Hi Dennis,
Try to find some old Nokia Pop-Port™ phones, which have FBUS in bottom connector (i.e 7210 or similar).
Other option is to use even older DCT-3 phones (i.e. 6110 or similar). Shouldn't be too hard to find suitable.
All you need info about Nokia FBUS/MBUS cables
True +3V to +5.5V RS-232 Transceiver, MAX3221-MAX3243
Edit:
N80 is USB featured Pop-Port phone and therefore not usable.
8210 OK, but FBUS test pads under battery.
BR,
-Gusse-
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