You'll see an incoming call notification screen, tap or click the call button to answer the new call, the call you're already on will be put on hold. If you don't want to answer it, tap the end call button to decline. If you're offline and you have Voice Messaging activated, the caller will be able to leave you a message, or you can set up call forwarding to forward incoming calls to another number.
Ready to learn more?
Troubleshooting issues with Skype call quality