Parking a call is the process of placing a call on hold in a way that you can then retrieve it from any other handset.
During a call press the softkey More till you see the softkey Park appear, press it.
On the handset you wish to retrieve the call from press the softkey Unpark and dial the number you placed the call on park from. Then press the softkey Unpark again to take the call.
Now the caller will still be on hold but the call will be connected to a different handset, when you are ready to resume the call press the softkey Resume.