Skype has been discontinued. Microsoft retired Skype on May 5, 2025. The Skype icon click-to-call feature described in this article no longer works, since there's no Skype app left to receive calls.
Looking for a way to add a direct contact link to your signature? See How do I add social media profile and page links? — WhatsApp is a good modern alternative for a click-to-chat/call icon.