If you're signed into Skype, you can receive calls. You'll see an incoming call notification screen where you can:
Select the Call button to answer the call as an audio call.
Select the Video button to answer with video. For incoming video calls, you can select to answer it as a video call or audio only.
Select the End call button to decline it if you don't want to answer. If you're offline and you have Voice Messaging activated, the caller will be able to leave a message, or you can set up call forwarding to another number. 
Select the Merge calls button to merge your current and incoming calls.
Android users have the option to decline calls with a message: Tap the
Message button, select from a list of common messages or type your own.
If you can't or don't want to allow access to your microphone and camera, you can also join a call just to listen in (using
Skype for Web only). When answering a call, select
Block or
Don't allow if Skype asks for permission to use your camera and microphone. Then, click the
Continue without audio or video button to join the call. Note that the other participants won't hear you or see your video, but you will appear in the participants list and be able to use chat.
Ready to learn more? How do I make a call in Skype? What can I do during a Skype call? What happens when someone calls me on Skype while I’m on another call? 