Voltage of smartphone audio jack
Depending on the standard used, the microphone pin will have a voltage (generally with a large resistance in series). This is needed to allow the microphone circuit to work. The large resistance in series is there so that when the microphone pin is shorted out by the headphone cable, it doesn't damage anything (almost no current can flow and the headphone is not damaged). Since the current draw of the microphone electronics is very low, when connected it does not cause a large voltage drop allowing the microphone to work.

Here is a similar question that might shed some light on this matter: 
Microphone voltage on a 3,5mm PC microphone/headphone jack