In order to setup a function key to be used to switch on or off call forwarding, on a Yealink handset, follow the steps below. 

1. Press the Menu button. 

2. Select Features

3. DSS Keys

4. Select the line key that you would like to change i.e. select Line Key #4, which is top right on the handset (T41P)

5. Type = Key Event

6. Key Type = Forward

7. Label = blank which displays ‘Forward’ (optional) you could set this if you wanted

8. Value = number to forward to, for example mobile 0444555666

5. Save

You should then see the top right button with the Label ‘Forward’, and it switches on or off on each press.