This circuit originally appeared at http://www.mobil.cz/telefon/triky/6110/zapojeni.html.

I can confirm that the above circuit works with Nokia Cellular Data Suite v1.2. The only change I made was to substitute 1N4148 diodes for D1 and D2 which are shown as Schottky. A hands free headset seems to be the easiest way to obtain a connector to fit the phone.