See also:

Adjustable level of each volume
...

Volume control
Volume control You can adjust the phone call volume, the voice volume and the voice guidance volume. Refer to “Volume control buttons”. ...

Taking a phone call and declining an incoming call
To take a phone call: Press the OFF hook switch . To put an incoming call on hold: Press the ON hook switch briefly. To decline an incoming call: Press the ON hook switch for more than 1.5 seconds ...