I’ve realized that we have a limitation indeed at the xwiki Syntax level. we’re missing a uri: prefix. Right now we have a url: one but that’s too limiting. See https://www.xwiki.org/xwiki/bin/view/Documentation/UserGuide/Features/XWikiSyntax/?syntax=2.1§ion=Links
Right now, when using the wiki syntax, the workaround is to use the path: prefix, as in:
[[label>>path:sip:+49XXXXXXXX]]
You need to do the same in the WYSIWYG editor and use “Path”.
Thanks
PS: Using URL is wrong since sip:+49XXXXXXXX is clearly not a URL.