Configuration => Catalog => Inventory:
Display Out of Stock Products => Yes
Manage Stock => No
This should allow to sell products without managing remained products in stocks
Otherwise just replace with
<?php if ($_product->isAvailable()): ?>
<p class="availability in-stock"><?php echo $this->__('Availability:') ?>
<span>
<?php
$qty = (int) Mage::getModel('cataloginventory/stock_item')->loadByProduct($_product)->getQty();
if ($qty<=0){echo $this->__('Call for Availability');}
else {echo $qty;} ?>
</span>
</p>
<?php else: ?>
<p class="availability out-of-stock"><?php echo $this->__('Availability:') ?> <span><? php echo $this->__('Call for Availability') ?></span></p>
<?php endif; ?>