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:
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