We can't support generic URI schemes as they would clash with our namespace separators (eg. [[callto:12345]] is a valid internal page id in the namespace callto). URL schemes on the other hand can be easily used. And the daringfirball link suggests that callto://12345
will work just fine. It probably makes sense to add callto to conf/schemes.conf
The alternative would be to specify interwiki links for this so users could use [[callto>12345]]. The benefit would be that we could even use an appropriate interwiki icon.