Possibly interesting recent discussion about tel:// vulnerabilities, although I don't think that applies in this case since we don't have a native app.

http://algorithm.dk/posts/rtfm-0day-in-ios-apps-g-gmail-fb-messenger-etc

• This should work after D10449 lands.
• Because this protocol is not common, I've left it out of the default set. You can use a command like phabricator/ $ ./bin/config set uri.allowed-protocols '{"http":true, "https":true, "mailto":true, "tel":true}' to add it.
• If you have existing links, run phabricator/ $ ./bin/cache purge --purge-remarkup to purge the caches.
• I'm only like 95% sure the implementation is correct since I don't have a convenient way to test it on a real device with telephony capabilities (iOS Simulator just fails when clicking a "tel:" link), yell if you're still seeing issues.