You might want to try this and see if it does what you want.

in admin -> catalog -> product type -> product general -> Show Quantity in Stock set to false.

Now you should be able to add quantities to your products but the X units in stock will not show up just the call for price.

I know this is not exactly what you had in mind but it as close as I can come to it.

As Kobra said, it will take a bit of custom code to get the "In Stock" message to show up.