How to disable Google Assistant?
This article applies to Android 9, Android 10 and Android 11.
On Android 9 and above Google Assistant may interfere with Zello processing headset buttons while in the background. To disable Google Assistant, follow these steps.
From system settings search for "Devices"
Select Google Assistant > Devices from the search results:
Select Wired headphones
Uncheck Get help from Google
You may need to open Zello app at least once before it regains ability to process wired headset keys while in the background.