The the problem is probably the standard 3.5mm jack. Smart phones use four pole 3.5mm jacks to add a mic to a headset.
There are two configurations used but the most common (read what Apple pushes) is to have the sleeve as mic in, ground on the adjacent ring, right audio on the next ring then left on the tip.
Pinout with list of phones
I know that most recent Samsung phones reverse ground and mic in. I actually built a small crossover cable to get around this issue.
According to this page the mic should work if the resistance is about 2.2k ohms.
I only assume this works based on the success of using my crossover cable to use a headset/mic with button designed for iPhone on my Galaxy S.
Thanks to a certain theBear on IRC's freenode (#electronics), I learned that devices generally test for the presence of a microphone by checking the resistance across the bottom two pins.
There doesn't seem to be a consistent convention for pinout, but generally mic and ground occupy the two rings closest to the sleeve
so the device tends to check for the resistance between these bottom two rings to determine whether a microphone is present.
for a standard stereo plug (L+R+G), The resistance should be practically zero as there is just one single ring that will be connecting the G and M
so the host says ' if the resistance is < X, then we assume no microphone. '
clearly 100 ohm is too small a value for X
doubling up to 200 did the trick
I still didn't get it working on an android phone even trying 300. So I don't know what the deal there is.
in case anyone wishes to pursue this further, the advice I got was to try putting a potentiometer between the microphone and ground and turning it until the host registers that a microphone is connected
if you launch the sound recorder on your android device and sing while you are twiddling, at some point the spectrograph will just cut out -- this means that the device now believes a microphone is connected externally
the last suggestion was to string a capacitor in series with this small resistor. however I don't have any idea how to calculate what value to use
As a final note, the ratio of R1:R2 determines the ratio of V_in:V_out
Best Answer
There is no 100% sure answer to your question. Because we (and maybe you?) don't know exactly what microphone you are talking about and how it is wired. There is more than one convention for wiring microphones to TRS 3.5mm plugs.
I would first try connecting just the tip of the mic connector to "Microphone" ring on the iPhone. And if that doesn't work, I would connect BOTH the ring and the tip together to the "Microphone" ring on the iPhone.
Ref: http://www.epanorama.net/circuits/microphone_powering.html#soundcard