Audio signal circuit. What is the 200 Ohm resistor doing in this circuit? REDRAWN
Your schematic reduces to







simulate this circuit &ndash; Schematic created using CircuitLab

with mic left, iPhone right, and the sleeves being ground, and Ring2 being the mic connection according to the CTIA standard. The rest of the connections (Ring on the mic, Ring1 and Tip on the iPhone side) are left unconnected.

Your microphone is an electret one, which probably means it's hard for the iPhone to detect the presence of the microphone simply by measuring the resistance across it.

If you imagine the mic away, then you just "shorted" the microphone input of the iPhone to its ground connection with about 1.7 kΩ resistance, which might just be in the right ballpark to tell the iPhone there's an external active microphone attached.

Of course, you don't just need the microphone to be there, you also need it to deliver sound. My guess is that the sensitivity (volts per sound pressure) of the microphone is relatively high, and that the sink impedance of the microphone input on the iPhone is relatively low: In that case, the array of resistors forms a current divider, which in turn brings the voltages at the iPhone to the level it can work with.