Ah,
So I attach the mic and port to one side, but the arduino output to the other. You are right though, I do need to drop some voltage, I believe that the mic input would need around 100mV whilst the arduino is kicking out 5V.
If I were to use a voltage divider with something like R1= 470 ohms and R2 = 10 ohms, giving 100mV out, would that isolate the voltage enough, but still allow the audio 'signal' to be interpited by the microphone input on the phone?
Just throwing these things out there as its been years since I've done any electronics!
I'll buy a transformer, but just interested to know if the voltage divider would do in a pinch?! Don't ask, don't learn!
Thanks for your help!