You will have several options while on a call:
Keypad (to the left by the Toolbar) to enter numbers during the call (e.g. multiple choice menus)
Loudspeaker to choose from a range of computer audio options
Hold to place the other party in a waiting room
End Call to hang up on the party
Mute to turn off your microphone
Transfer (blind) to transfer a caller to an internal or external contact
Attended Transfer to introduce a call before transferring
Add Call to add another contact or user into a conference call