On some phones like the Grandstream Budgetone you can pick up the phone, hit "menu", and dial the 0-padded 12-digit IP address of the other phone followed by "send" and the other phone will ring. Both phones will obviously need to have real IP addresses.
If the phones are behind NAT boxes, you can still make free, direct calls. You just need to use a sip server someplace to help you make the initial contact. Something like Free World Dial
What I find interesting is that some large companies and organizations are starting to publish their SIP contact details in dns. To dial an extension at MIT I could go directly across the net without using any middleman.
hmm, I must admit this is a bit over my head, I'm using a FDX-840 and a normal hand set analogue phone connected to his, I thought or was thinking maybe picking up the handset and punching in a IP rather than a phone number...