Some Cellphone has built in Echo cancellation function and causes interference with our echo cancellation.
Press 80# from your home phone key pad while off hook(should hear an acknowledge tone) then reboot Cell2Jack (power off/on).
Also, Some cellphone only supports Bluetooth earphone but not fully support Bluetooth MIC.
You need lower your cell phone Bluetooth audio volume below 25% after the call connected.
You only require to setup once.
Also, Bluetooth audio volume setting won't affect your cellphone normal audio volume.
Note: click volume bar to open audio volume setting.