Can I use another applications during an active call? (Series 40 phones)

Yes. During a call, press the selection key Loudspeaker (or Loudsp.). If the screen or keypad is locked, unlock it first. Then you can continue the call and at the same time open another application via Menu or Options > Menu.

If you are in application view when receiving a call, answer the call, then go to the idle screen to press the Loudspeaker (or Loudsp.) button.