How to mark-up phone numbers?

Question

I want to mark up a phone number as callable link in an HTML document. I have read the microformats approach, 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 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:

Answer 1

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>