Skype — the high-fidelity voice over IP chat/phone service — supports the “callto:” URI scheme. What this means is that in the same way a “mailto:” link can be used to initiate a new email message from a web page:
<a href="mailto:firstname.lastname@example.org">Send me an email</a>
you can use a “callto:” URI to initiate a Skype call. To send a message to someone with the Skype username “example”:
<a href="callto://example">Call me on Skype</a>
Using the “SkypeOut” feature, you can use Skype to make phone calls, which for international calls can be remarkably cheaper than direct phone-to-phone connections. Just use the phone number in place of the Skype username, prefixed by an international calling code (“+1” within the U.S.):
<a href="callto://+12125551234">Call my phone from Skype.</a>
Except that last example won’t work from Safari: “callto://username” URIs work just fine; “callto://phonenumber URIs don’t work at all. You click them and nothing happens.
The solution is not to use the slashes after the “callto:” scheme:
<a href="callto:+15558675309">Jenny, you got my number.</a>
and they’ll work just fine. I have no idea which format is “correct”, but given that “mailto:” URIs have never used slashes, my money is on the no-slash format being canonical. The most authoritative source I could find is this MSDN “CallTo URL” developer documentation, which indicates the slashes should not be included.
Firefox (and other Mozilla-derived browsers, such as Camino) work just fine with “callto:” URIs with or without the “//”; Safari allows for the “//” if the recipient is a Skype username, but not if it’s a phone number.
So if you’re working on Skype integration — or, as we call it in the Web 2.0 world, a “Skype mash-up” — don’t use slashes in your “callto:” URIs if you want them to work in Safari.