There are two ways to have click-to-call links in HTML
<a href="wtai://wp/mc;+1800229933</a>
WTAI style (Nokia, others)<a href="tel:+1-800-275-2273">Call Apple Customer Support at 1-800-275-2273</a>.
TEL style (Apple)
How one can
detect which format is supported by current user agent in Javascript?
Is it possible to do the detection without relying the user agent string
More info