I've connected a speaker to the gsm shield with the SparkFun amplifier board. Works fine (although volume is still weak). But it is ok.
Next step was to connect a electret microphone to the board (M1P and M1N). This does not work. I suppose I need some extra hardware here. The gsm shield doc mentions some hardware (3.9.3. Earphone interface connection) but this is for me not clear at all.
Can someone explain me what I need to connect a mic to the gsm shield. Objective is to be able to make voice calls.