Thank you Kevin for answer! Yes, I've created new ticket through "feedback" some time ago (FB16279493 (It's not possible to access system PTT UI when app is backgrounded and "Always" location access is activated for app)) but didn't get any answer, so decided to ask about the same here.

And just to clarify, app waits both didBeginTransmitting and didActivate notifications before try to access mic. Also, the way how PTT start sound is distorted, sounds for me very similar to what I hear if I start some audio playback with .playback category and do switch to .playAndRecord category while audio is still playing...

@DTS Engineer Thank you for explanation. It has meaning and helpful. But unfortunately I see problem with .fullDuplex and without any headset (not wired, nor wireless) attached... Anyway, It looks like I have to implement basic app reproducing issue to be sure it's not something in our current app. I'll back with results later. Thanks!