Skype call button

Are you referring to this button?
https://www.skype.com/en/developer/create-contactme-buttons/

If so, I’m afraid you won’t be able to add it to the list of Jetpack sharing buttons as it requires some JavaScript. You also need to specific your Skype username to be able to generate the button.

I would recommend that you generate your own button and place its code in your theme’s single.php template file, just below the the_content function.

I hope this helps!