Fairphone 2 (A9). Audio jack 3,5 mm compatibility
There are different standards in using the 4-pins of a headset with mic (and remote). Here you can find the explanation about the different types and with a simple measurement you can check what (wired) headset you have:
The FP2 uses the standard AHJ=CTIA (ground=2) - on the right image above.
We do not have the capacity to test headphones and we are not in the position to endorse the use of any brands or types of headphones. Our awesome community helped to make a list of headphones that have been tested with Fairphone 2.
Your device does not work with your Fairphone 2
If you use compatible headphones (see above) and the sound is not working, please check that your head jack is not damaged. Try at least 3 different headphones to deduct if it is really the head jack of the FP2 causing the problem, and not a broken headphone.
Also, be sure your headset/phones are suitable for Android devices. Headsets made for other phones than Android-based phones might not work.
For any wireless device, also consult the Bluetooth-issues article.