Iconic Flux checkout - validate phone number on step change.

functions.php

<?php
/**
 * Iconic Flux checkout - validate phone number on step change.
 *
 * @param array $messages The inline errors.
 *
 * @return array
 */
function flux_checkout_custom_inline_error_message( $messages ) {

	$value = $_POST['fields']['billing_phone']['value'] ?? false;

	if ( ! $value ) {
		return $messages;
	}

	if ( preg_match( '/^07/', $value ) ) {
		return $messages;
	}

	$messages['billing_phone']['message'] = 'Please enter a valid phone number that starts with 07';
	$messages['billing_phone']['isCustom'] = true;

	return $messages;
} 
add_filter( 'flux_checkout_check_for_inline_errors', 'flux_checkout_custom_inline_error_message' );