Remarkup doesn't recognise [[ http://tools.ietf.org/html/rfc3966 | `tel:` URIs ]] in hyperlinks even if they are added to setting `uri.allowed-protocols`.
This is because they don't match the regular expression in [[ https://secure.phabricator.com/diffusion/PHU/browse/master/src/markup/engine/remarkup/markuprule/PhutilRemarkupDocumentLinkRule.php;e1c7a6ee32d272762d70cf63ef0b9c974f84a300$103 | PhutilRemarkupDocumentLinkRule::markupDocumentLink ]], which treats the similarly `//`-less `mailto:` scheme as a special case.