One of the reasons I wanted to get a Cisco 504G was to test the FreePBX/Asterisk Paging & Intercom module. With every new phone release its always important to test the feature set before deploying to clients.
The paging and intercom work the same way as the SPA942 models. Just need to enable "Auto Answer Page" and remove any vertical service codes that might conflict with FreePBX paging/intercom codes.
There is a new multicast feature that enables paging directly from the phone - need to look into this more.