Skype for Mac OS X
How to answer a call
If someone is calling you, you will hear the sound of a telephone ringing and a floating call window will appear on your screen.
Click on “Answer” to accept the call or “Reject” to decline it. As soon as you decide to answer a call, a two-way voice conversation will begin and the duration of the call will be visible. Either party can end the call at any time by clicking the red “Hang up” button.