Why Can't the Other Person Hear Me during a Call
There may be some causes for this issue.
Reason 1. There is only one microphone working when you using two earbuds - the one of the earbud which connected with the phone.
So when you share one earbud with other people, please confirm the microphone on which earbud before using it for talking
How to confirm the microphone on which earbud?
1. The earbud you hear "Connected" voice prompt
2. If you miss the voice prompt, please check the Bluetooth list on your device, the one which shown as "Connected"
Reason 2. If you answer calls on your mobile device instead using the earbuds. Your device may use the the built-in microphone of your phone as default.
So please choose the audio device as Avantree TWS106R or Avantree TWS106L (depends on the mic on which earbud) when on a call.