iMac headphone's external mic not work I have 5 iMac in my office. We use skype for conversation. I have used almost 5 different brand's headphone but headphone's external mic not work at all. 

I have checked sound settings. In the sound settings under "Input" section, when I select "Line in" for my headphone the mic does not work. But when I select "internal microphone" it works.

The same headphones are working perfectly on windows system. 

Does iMac require any special headphones?
iMac line-in for Microphone is not powered! 

Means you can not use a standard microphone. 

It has to be one with power-source build in to the microphone. 

In your case there are 3 choices.

Buy a line-in microphone with preamp.

Get a USB type Microphone (headset)

Cost reducing alternative:
If you have a Bluetooth headset for your phone (which you should if you making phone calls in the car), you can use it it 2.