IPhone – When EarPods with microphone are attached, does iPhone use internal or external mic

You cannot directly plug your headphones into the Audio Input jack and have it work, because, the microphone/button signal rides on the third ring, and the audio input jack only connects the first two. You have to have specially designed hardware to connect to that third ring. This is currently achieved via the Headphone/Audio Output ports of devices.

Only the iPhone/iPod Touch/iPad, 2010 MacBook Pros, and the modern body (smaller height) Mac Minis are the only pieces of hardware I am aware that do. Anything older will not support the microphone/button/volume buttons at all.

You can solve this problem physically with an extension cable.

Check the tip of the extension cable (the male end) for the number of rings. If it only has 2, it is only recognized as a pair of headphones. The third ring carries microphone audio which you'll see on the end of the headset cable.