Default [Q] Microphone mute in phone/Skype but not other apps

I have a non-rooted Asus Zenfone 5 with recent OTA firmware. When I attempt a normal voice call or Skype call the microphone is muted i.e. I can hear the caller but they can't hear me.

Other microphone related activies such as Sound Recorder, Google voice search and Keep voice note work fine. Using a microphone headset doesn't make any difference.

I've searched through every setting imaginable and tried a few mute apps from Google Play and no luck.

Any feedback is appreciated.