To mute means that e.g. the customer cannot hear you and you cannot hear the customer. Normally mute is turned on automatically when you initiate an enquiry call.
For mute on/off you can
- click on  , ,
- select Menu | Contact handling | Mute on/off, or
- press F8.
You can see if mute is on or off by the colour of the mute button.
- 
                                             Half the icon is red. Mute is on. Half the icon is red. Mute is on.
- 
                                             Mute is off. Mute is off.
- 
                                             The button is completely grey. The feature is not available at the moment. The button is completely grey. The feature is not available at the moment.