// Custom validation for Billing Phone checkout field 
add_action('woocommerce_checkout_process', 'custom_validate_billing_phone'); 
function custom_validate_billing_phone() { 
    $is_correct = preg_match('^((8|\+7)[\- ]?)?(\(?\d{3}\)?[\- ]?)?[\d\- ]{7,10}$', $_POST['billing_phone']); 
    if ($_POST['billing_phone'] && !$is_correct) { 
     wc_add_notice(__('The Phone field should be <strong>between 6 and 20 digits</strong>.'), 'error'); 
    } 
}