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:

1-847-555-5555

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

As one would expect, WebKit's support of tel: extends to the Android mobile browser as well - FYI

I keep this answer for "historic" purpose but don't recommend it anymore. See @Sidnicious' answer above and my Update 2.

Since it looks like a draw between callto and tel guys, I want to throw in a possible solution in the hope, that your comments will bring me back on the way of light ;-)

Using callto:, since most desktop clients will handle it:

<a href="callto:0123456789">call me</a>

Then, if the client is an iPhone, replace the links:

window.onload = function () {
  if (navigator.userAgent.match (/iPhone/i)) {
    var a = document.getElementsByTagName ("a");
    for (var i = 0; i < a.length; i++) {
      if (a[i].getAttribute ('href').search (/callto:/i) === 0) {
        a[i].setAttribute ('href', a[i].getAttribute ('href').replace (/^callto:/, "tel:"));
      }
    }
  }
};

Any objections against this solution? Should I preferably start from tel:?

Mobile Safari (iPhone & iPod Touch) use the tel: scheme.

How do I dial a phone number from a webpage on iPhone?

RFC3966 defines the IETF standard URI for telephone numbers, that is the 'tel:' URI. That's the standard. There's no similar standard that specifies 'callto:', that's a particular convention for Skype on platforms where is allows registering a URI handler to support it.

this worked for me:

1.make a standards compliant link:

<a href="tel:1500100900">

2.replace it when mobile browser is not detected, for skype:

$("a.phone")
    .each(function()
{ 
  this.href = this.href.replace(/^tel/, 
     "callto");
});

Selecting link to replace via class seems more efficient. Of course it works only on anchors with .phone class.

I have put it in function if( !isMobile() ) { ... so it triggers only when detects desktop browser. But this one is problably obsolete...

function isMobile() {
    return (
        ( navigator.userAgent.indexOf( "iPhone" ) > -1 ) ||
        ( navigator.userAgent.indexOf( "iPod" ) > -1 ) ||
        ( navigator.userAgent.indexOf( "iPad" ) > -1 ) ||
        ( navigator.userAgent.indexOf( "Android" ) > -1 ) ||
        ( navigator.userAgent.indexOf( "webOS" ) > -1 )
    );
}

I used tel: for my project.

It worked in Chrome, Firefox, IE9&8, Chrome mobile and the mobile Browser on my Sony Ericsson smartphone.

But callto: did not work in the mobile Browsers.

I would use tel: (as recommended). But to have a better fallback/not display error pages I would use something like this (using jquery):

// enhance tel-links
$("a[href^='tel:']").each(function() {
    var target = "call-" + this.href.replace(/[^a-z0-9]*/gi, "");
    var link = this;

    // load in iframe to supress potential errors when protocol is not available
    $("body").append("<iframe name=\"" + target + "\" style=\"display: none\"></iframe>");
    link.target = target;

    // replace tel with callto on desktop browsers for skype fallback
    if (!navigator.userAgent.match(/(mobile)/gi)) {
        link.href = link.href.replace(/^tel:/, "callto:");
    }
});

The assumption is, that mobile browsers that have a mobile stamp in the userAgent-string have support for the tel: protocol. For the rest we replace the link with the callto: protocol to have a fallback to Skype where available.

To suppress error-pages for the unsupported protocol(s), the link is targeted to a new hidden iframe.

Unfortunately it does not seem to be possible to check, if the url has been loaded successfully in the iframe. It's seems that no error events are fired.

Since callto: is per default supported by skype (set up in Skype settings), and others do also support it, I would recommend using callto: rather than skype: .

Although Apple recommends tel: in their docs for Mobile Safari, currently (iOS 4.3) it accepts callto: just the same. So I recommend using callto: on a generic web site as it works with both Skype and iPhone and I expect it will work on Android phones, too.

Update (June 2013)

This is still a matter of deciding what you want your web page to offer. On my websites I provide both tel: and callto: links (the latter labeled as being for Skype) since Desktop browsers on Mac don't do anything with tel: links while mobile Android doesn't do anything with callto: links. Even Google Chrome with the Google Talk plugin does not respond to tel: links. Still, I prefer offering both links on the desktop in case someone has gone to the trouble of getting tel: links to work on their computer.

If the site design dictated that I only provide one link, I'd use a tel: link that I would try to change to callto: on desktop browsers.

Using jQuery, replace all US telephone numbers on the page with the appropriate callto: or tel: schemes.

// create a hidden iframe to receive failed schemes
$('body').append('<iframe name="blackhole" style="display:none"></iframe>');

// decide which scheme to use
var scheme = (navigator.userAgent.match(/mobile/gi) ? 'tel:' : 'callto:');

// replace all on the page
$('article').each(function (i, article) {
    findAndReplaceDOMText(article, {
        find:/\b(\d\d\d-\d\d\d-\d\d\d\d)\b/g,
        replace:function (portion) {
            var a = document.createElement('a');
            a.className = 'telephone';
            a.href = scheme + portion.text.replace(/\D/g, '');
            a.textContent = portion.text;
            a.target = 'blackhole';
            return a;
        }
    });
});

Thanks to @jonas_jonas for the idea. Requires the excellent findAndReplaceDOMText function.

I use the normal <a href="tel:+123456">12 34 56</a> markup and make those links non-clickable for desktop users via pointer-events: none;

a[href^="tel:"] {
    text-decoration: none;
}
.no-touch a[href^="tel:"] {
    pointer-events: none;
    cursor: text;
}

for browsers that don't support pointer-events (IE < 11), the click can be prevented with JavaScript (example relies on Modernizr and jQuery):

if(!Modernizr.touch) {
    $(document).on('click', '[href^="tel:"]', function(e) {
        e.preventDefault();
        return false;
    });
}