Source code for https://formidable-masterminds.com/strip-non-numeric-characters-from-phone-number-with-jquery/

remove-non-numerics.js

<script>
jQuery(document).ready(function($) {
    "use strict";
    /* Format link phone number */
    $("#field_devdir_country_code, #field_devdir_primary_contact_phone").on("change", function() {
        var link_number = $("#field_devdir_country_code").val() + $("#field_devdir_primary_contact_phone").val();
        $("#field_devdir_link_phone_number").val( link_number.replace(/\D/g,'') );
    });
});
</script>