Mask a custom user field value displayed in the Member Directory for non-members .

pmpromd-mask-phone-for-non-members.php

<?php
/**
 * Mask the phone number displayed in the member directory for non-members.
 *
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
function my_pmpromd_mask_phone_for_non_members( $value, $element, $pu, $displayed_levels ) {
	if ( ! pmpro_hasMembershipLevel() && $element === 'member_phone' ) {
		$value = '***************';
	}
	return $value;
}
add_filter( 'pmpromd_get_display_value', 'my_pmpromd_mask_phone_for_non_members', 10, 4 );