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="https://stackoverflow.com/questions/1164004/tel:+18475555555">1-847-555-5555</a>