Yeah, that’s a known bug with the new Outlook rendering. Easiest workaround is to wrap the phone number in a regular <a> tag but make the href look like a normal URL, e.g. href="https://example.com/tel/0000000000", or just drop the tel: link and show the number as plain text. Outlook strips or rewrites tel: links right now, so not much you can do until they fix it.