While in conversation, pressing the “Hold” button (shortcut F5) will put the remote end on hold. The Call button in the Activity Panel will change it’s status from Green to yellow and the Hold button will change it’s status from White to Orange, but just for that call.

The call will remain in Hold till you resume it.
To Resume a call select it from the Activity Panel and then press the hold button (shortcut F5).
If you have an active call in progress and you initialize an other call (es. an extension) or you answer to an incoming, The active call will be placed in Hold.