We can use the
tel: URL scheme for phone numbers in HTML. Just like the
mailto: URL scheme will open the default mail application will the
tel: start a telephone call. If the HTML page is viewed on a mobile phone and we select a link with the
tel: scheme we can immediately call the number following the scheme. On a desktop computer a VOIP call will be initiated.
We can use hyphens in the phone number for readability, they will be ignored when the call is made. For example the imaginary phone number 123456789 in the Netherlands can be used as shown in the following HTML snippet:
<a href="tel:+31123456789">Call us</a>
More information is available at http://www.ietf.org/rfc/rfc3966.txt.