I have also two simple HeadSets, which are not properly working with the FP2 but did it like a charm with the FP1 and another old Nokia phone.
I made some more research I want here to share and which I also sent to support.
The 3.5mm headsets are 4-wired (2xstereo sound, 1 mic, 1 ground) with a call button.
Strange behaviour includes:
- If I plug in the headset slowly I** can’t see any symbol** in the notification bar (maybe normal behaviour of Android 5.1 in contrast to 4.2, which clearly showed me If I have plugged in a headset or a headphone)
- During plugging in, I can feel different “snaps” according to the 3 black rings of the jack-stick which are connected with the outer contact of the jack.
a) if it connects to the contact between 2nd and 3rd ring: my right headphones of the set begins to make clicking noises continously
b) if it connects exatly the 3rd ring there is nothing to hear
c) if it connects to the last contact after the 3rd ring: the clicking noise at the right headphone is back, just quieter - If I only play music through the headset I basically CAN hear the music, but in a very bad quality. Only during pressing the CALL-BUTTON the sound is as expected!! Releasing the button, sound again bad!
- If I try to call through the headset the mic seems to work - but in an unexpected way:
The headphones itself do not work (or extrem quietly). But during PRESSING the CALL-BUTTON I can instantly clearly hear everything correct (as Nr. 3 above). The MIC is working quite OK, but DURING PRESSING the CALL-BUTTON, the MIC is disabled.
Thus it is not possible to make a normal call !! - During plugging-in any headphone/-set one should care about to fully plug-in the jackstick, as the last 1mm needs some stronger pushing… see pictures
- If I plug-in the headset, the software makes different things.
-sometimes a grey screen appears just announcing “Initializing” in the middle
-sometimes the music player continues to play (could be intended behaviour)
-sometimes the music player does not continue to play
-sometimes it appears the screen from “Google Now - please speak now”
forum threads in connection: here, here, here and the semi-official list here
For me it seems that there is any contacts inside the FP-jack wrong connected.
OR: my headset is not suitable for the FP2. BUT: Why it worked without problems with the FP1 and another Nokia-phone?
Plus the (hopefully) software-side is not programmed fully functional…
Playing Music through other headphones (without any mic) works well!
Any comments ???
Cheers, Robert
My headsets are:
Nokia WH-102 HS-125: 
and
B7 ZTE Blade: 