Text on out of stock items. Editing for all products.

[benji86]

Hi,
I have an online store running with over 1700 products, so I cannot go editing each product as it would take too much time.

I have it so items can be ordered while out of stock as well as in stock, which is how I want it.
However sometimes stock can be wrong, so I would like for out of stock items to display 'Call for Availability' on the product page.

I have been able to edit a single product using the Quantities > Displayed Text when Out of Stock.
However this is impractical as I need it to do this on ALL Products when out of stock.
Is there a way to edit the theme or a page?

Thanks in advance.