The ACF field is "Text" type (as far as I know, there's no phone type).
The "tel:" is manually typed before the Oxygen shortcode in the URL box of the "Link Wrapper" element. I didn't see any other way to create a tel: link using the Link Wrapper. I also did the same with a "Text Link" element.
Originally from #898 via @Green-Zero-Development
Confirmed and reported internally via https://github.com/soflyy/oxygen/issues/4045.
A temporary workaround is to use the Meta / Custom Field Data Point instead of the Advanced Custom Field Data Point.