iMac headphone's external mic not work
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.