When you answer the call in standby screen (home screen), press the soft key Loudspeaker (or Loudsp. or Activate loudsp.). You can continue talking on the phone and meantime access to an application by pressing menu.
If you are in an application view when receiving a call, long press Menu key and select Telephone from Active applications window, in Telephone screen press Loudspeaker button.
To switch between Standby, Telephone application and other applications, long press Menu key.