Hi Tony,

are you using a 8210 cell phone for your project?
As the three pole plug seems to be used there only.
Do you have a common ground on the phone and your circuit?
Then you should check if MIC- is on ground potential.
Futhermore I don't think you will need a "strong" signal, as a microphone would not deliver more than 100mV (guess) signal.
Make sure not to overdrive the input.

Regards,
Ingo