Okay, may sound daft and there appears to be very little on this for me to research and understand.
this link explains a little on what I have found.
In theory from what I read:-
an ardunio pwm pin wires to (speaker/ microphone terminal on the gsm module), I cant see why it be a speaker though as sending signals from the gsm to another mobile phone. Not listening locally, so speaker pointless, but microphone would be incoming sound?
So a resistor is required to mimic the resistance of the speaker/ microphone and the pwm pin delivers the audio from a file.
Before I go out and get a sim card module does anyone know where i can find more info on this, resistor value and confirm if it is the speaker or microphone I need to use or if its complete and utter rubbish.