You can make the phone number "clickable/callable" through the LINK button. Highlight the number or text you want to activate. Click on the "link" tool. There is an option to put in a phone number and that makes it a live action.
If you are talking about a Website Builder site, then Lynn is exactly correct. You just need to add the button or icon and click the link option. One of the choices says "Phone number". Any Sitebuilder Plus sites, this would need to be done via HTML.