People couldn’t hear me when I called from a Nexus 5x phone, after a security update for Android 8.1.0 early October 2018.

This was fixed by wiping the System Cache Partition. This will not delete your files, but it’s best to backup everything.

  1. Switch off the device
  2. Press & hold the Volume Down button and press & hold the Power button simultaneously until the phone turns on. You’ll see the word Start with an arrow around it.
  3. Press the Volume Down Button twice to highlight Recovery mode.
  4. Press the Power button to start Recovery mode. An android robot will show up with a red exclamation mark sign.
  5. While holding the Power button, short press and release the Volume Up button, a menu will show up. (do NOT hold the volume Up button as this will reboot your device!).
  6. Use the Volume buttons to scroll to Wipe cache partition and press the Power button to select it.
  7. After this, select Reboot device from the menu with the Power Button.

That fixed it for me.