ClearSounds CS-A300 DECT 6.0 Mode D'emploi page 10

sure you have turned the speakerphone off.
Using a Headset with your A300
You can use this telephone hands-free when you connect a 2.5mm
headset (purchased separately). To connect, plug the headset into the
HEADSET JACK on the left side of the handset. While in the Headset
Mode, the volume of the headset is controlled by the telephone and
the handset's earpiece and microphone are disconnected.
Using the Soft Key and Menus
In the LCD screen you will see the Menu and TAM options for setting
up and using your A300.
While in standby mode you will use the Soft Keys to select which
function from the Menu and set-up your phone.
10 – ENGLISH