To make a telephone number clickable on a smart phone you simply need to add a little HTML code.
Example:
<a href=“tel:+15122322576“>512-232-2576</a>
or
<a href=“tel:+15122322576“>Click to Call the Web Team!</a>
Note the syntax is similar to email addresses: <a href=”mailto:cofawebmaster@austin.utexas.edu”>cofawebmaster@austin.utexas.edu</a>
mailto: and tel: are both protocols.
Learn More
CSS Tricks goes into extensive detail in the article, The Current State of Telephone Links.