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's MIC. Cellphone's MIC is still on while using Bluetooth's MIC and cause distortion of audio.
You need lower your cell phone's Mic audio volume to min (see the picture on the right) to avoid interference.
You only require to setup once.
If audio is still bad, try use your home phone away from your cellphone (make sure your cellphone's mic won't pickup your conversation.)