Start a call
Tap the channel or direct message name to expand the Channel or Conversation Info pane. Next, tap Start a call from the Actions Menu.
End a call
Press the red End button.
Answer a call
Tap Answer from the toast notification or on the call message in the channel or direct message where the call is taking place.
Decline a call
Press Dismiss on the toast notification. If you dismiss the notification, it will appear in the notification shade. You can choose to answer or reject the call from there, as well.