When someone calls and you’re already on a call, you will see the Incoming call window on your screen.
You can choose to decline, answer, or add the call to the existing call:
If you have Skype
Credit or a subscription,
you can set
up call forwarding to forward incoming calls to another
number or Skype contact, or send the call to voicemail.
t
o
answer the call. This will put the current call on
hold, and you will need to click 
in the sidebar to switch between calls.
to add the call to the current call.