To park a call, click the P icon from the toolbar while on an active call.
You can choose to park the call against your own extension or enter another extension to park the call against. The client handles entering the * code and extension for you. If the call goes unanswered for too long, the system will ring you back with an incoming call.
To retrieve a call, use the top-level menu by choosing Calls->Retrieve Parked Call. Choose the phone to use and the client will place the appropriate call. When you hear the prompt asking you to enter the extension, click on the Dialpad icon and enter the # digit if the call was parked against your own extension, or the extension number that the call was parked against followed by #.