Users must sometimes leave their office, but you want their calls to go wherever they go.
The webphone function allows to deal with that type of situations but in some cases, the user cannot work with the webphone (for instance no headset).
You can then add their mobile or fixed phone number to call or be called, and continue your usual activities.
To do so, you must go to "Manage users" and select the user account to change (See How to configure a user).
You then just have to add the phone number in the "Phone. office" section; don't forget to save changes.