From http://inchoo.net/ecommerce/magento/user-friendly-product-availability-message-in-magento/ Como exibir a mensagem de produto fora de estoque.

product-out-of-stock.php

<?php // Check to see if the item is in stock
  if($_product->stock_item->is_in_stock == 1):
    echo $this->__('Availability') ?>: <strong><?php echo $this->__('In stock') ?></strong> ?>
  <?php else: ?>
    <strong><a href="<?php echo Mage::getBaseUrl(Mage_Core_Model_Store::URL_TYPE_WEB) ?>contacts/" title="<?php echo $this->__('Email') ?>"><?php echo $this->__('Email') ?></a></strong> <?php echo $this->__('or Call for Availability') ?>&nbsp;1-222-333-444
  <?php endif; ?>