As root, make a file /etc/apt/sources.list.d/skype-unstable.list
The content of this file is this line:
deb [arch=amd64] https://repo.skype.com/deb unstable main
Put a return at the end of the line, and save it. Then
apt update && apt install skypeforlinux
If there's a public key issue, it will show the needed key value.
If you want a great opensource encrypted chat and voice call package, take a look at
https://wire.com . Wire runs great on my siduction system. But your contact has to be using it too -- not compatible with skype.