Most modern devices support the tel: scheme. So use <a href="tel:555-555-5555">555-555-5555</a>
and you should be good to go.
If you want to use it for an image, the <a>
tag can handle the <img/>
placed in it just like other normal situations with : <a href="tel:555-555-5555"><img src="path/to/phone/icon.jpg" alt="Call 555-555-5555" /></a>