On a Yealink handset you can place an active call on hold. Only one active call can be made at any one time, so if you need to make another call during the call you are currently on, you will first need to place them on hold.
TO PLACE A CALL ON HOLD
Press the Hold soft key during the call
The LCD screen will then show that the call is on hold.
TO RESUME A CALL
Press the Resume soft key
MULTIPLE CALLS ON HOLD
If multiple calls have been placed on hold:
Press 
or 
to switch between the calls and press the Resume soft key to retrieve the desired call.