cybor The correct syntax would probably be
[123 456 789](tel:+41123456789) or
[url=tel:+4112345678]test[/url] for a phone number url, but neither seem to work.
tel: is probably not a protocol recognized by the post parser.
While not a solution in itself, I know that on Android (and maybe others ?) you can select any number on a page and it will automatically suggest calling it, even if it's not formatted as a link. So if you make sure the phone number is formatted correctly, many users will at least be able to use that feature.