I don't use Skype for my VoIP service but they all work the same, or should do. The settings should be set automatically in a normal scenario however, check your settings.
Click Web and Email Protectioon > Firewall
Click Internet Connections for Programs to expand
Click the word 'Program' at the top to rearrange them alphabetically.
Go down the list and look either for Skype by name or 'Client to make VoIP calls
Make sure it has Full access using the Edit button if necessary.
If that doesn't help try switching Netguard off while you are in that area, it helps with mine.
It also helps to go into the Skype Settings to make sure the correct sound devices are picked by the programme.
If you still have trouble, Technical Support is a free phone call away and linked under Useful Links at the top of the page, or you can use the free online chat, your choice.