Phone number constraints have been discussed in the forum before, e.g. Phone number constraint. You may also find hints in the documentation, see https://docs.getodk.org/form-regex/ (and https://support.kobotoolbox.org/restrict_responses.html#:~:text=KoBoToolbox%20supports%20regex%20to%20control,valid%20email%20id%20etc.)
And you may find more with the search function, e.g. https://forum.getodk.org/search?context=topic&context_id=54445&q=phone%20number%20constraint%20regex&skip_context=true.