How can I make direct phone-to-phone VoIP calls without Internet (on LAN or Wi-Fi)?
While I've not tried to use my phone like this myself, I found the question interesting enough to do a little googling.  The best search term seemed to be android wifi|bluetooth walkie talkie.

The most promising free candidate I found looks to be Android Intercom - direct phone to phone communication over wifi or BT without an intermediary server or telco.  The most promising paid for app seems to be WiFi Talkie.