iSymphony can be used to place calls on hold, or to retrieve calls from being held. There are two ways to do so:

Using the Hold Action Button

The Hold action button allows you to place any of your calls on hold, or retrieve calls from being held. To use it, click the Hold button in the action panel at the top of the iSymphony panel. Select the call you want to place on hold, and click Hold at the bottom of the popup. 

Using the Hold Call Button

Active calls that appear in the My Stream widget have a button that allows you to place them on hold easily.