How to mark-up phone numbers?

The tel: scheme was used in the late 1990s and documented in early 2000 with RFC 2806 (which was obsoleted by the more-thorough RFC 3966 in 2004) and continues to be improved. Supporting tel: on the iPhone was not an arbitrary decision.

callto:, while supported by Skype, is not a standard and should be avoided unless specifically targeting Skype users.

Me? I'd just start including properly-formed tel: URIs on your pages (without sniffing the user agent) and wait for the rest of the world's phones to catch up :) .

Example:

<a href="tel:+18475555555">1-847-555-5555</a>

My test results:

callto:

• Nokia Browser: nothing happens
• Google Chrome: asks to run skype to call the number
• Firefox: asks to choose a program to call the number
• IE: asks to run skype to call the number

tel:

• Nokia Browser: working
• Google Chrome: nothing happens
• Firefox: "Firefox doesnt know how to open this url"
• IE: could not find url

The best bet is to start off with tel: which works on all mobiles

Then put in this code, which will only run when on a desktop, and only when a link is clicked.

I'm using http://detectmobilebrowsers.com/ to detect mobile browsers, you can use whatever method you prefer

if (!jQuery.browser.mobile) {
    jQuery('body').on('click', 'a[href^="tel:"]', function() {
            jQuery(this).attr('href', 
                jQuery(this).attr('href').replace(/^tel:/, 'callto:'));
    });
}

So basically you cover all your bases.

tel: works on all phones to open the dialer with the number

callto: works on your computer to connect to skype from firefox, chrome