As the feedback post suggests, Skype offers the option to have encrypted conversations (chat & calls) using the Signal protocol. However, it's not the detault. If you're using Skype and this is something you'd like added, feel free to upvote the feedback request.

https://feedbackportal.microsoft.com/feedback/idea/0b770e9e-bca0-ed11-a81b-002248519701