I want to mark up a phone number as callable link in an HTML document. I have read [the microformats approach](http://microformats.org/wiki/hcard-faq#How_do_you_mark_up_a_phone_extension), and I know, that the `tel:` scheme would be standard, but is quite literally nowhere implemented.

Skype defines, as far as I know, `skype:` and `callto:`, the latter having gained some popularity. I assume, that other companies have either other schemes or jump on the `callto:` train.

What would be a best practice to mark-up a phone number, so that as many people as possible with VoIP software can just click on a link to get a call?

Bonus question: Does anyone know about complications with emergency numbers such as 911 in US or 110 in Germany?

Cheers,

**Update:** Microsoft NetMeeting takes `callto:` schemes under WinXP. [This question](https://stackoverflow.com/questions/1009181/url-scheme-for-phone-call) suggests, that Microsoft Office Communicator will handle `tel:` schemes but not `callto:` ones. Great, Redmond!

**Update 2:** Two and a half years later now. It seems to boil down to what you want to do with the number. In mobile context, `tel:` is the way to go. Targeting desktops it's up to you, if you think your users are more Skype people (`callto:`) or will more likely have something like Google Voice (`tel:`) installed. My personal opinion is, when in doubt use `tel:` (in line with @Sidnicious' answer).

**Update 3:** User @rybo111 noted, that Skype in Chrome has meanwhile jumped on the `tel:` bandwagon. I cannot verify this, because no machine with both at hand, but if it's true, it means we have finally a winner here: `tel:`