<a href="tel:5558675309">555-867-5309</a> That should work.
If you need to enter a popup dialog to ask the user “Are you sure you want to call?” Just in case s / he clicked it unintentionally.
<a id="call" href="">555-867-5309</a>
Use the call identifier to open the dialog by clicking OK to dynamically place the data in the href attribute.