You are correct, several Sprint / Nextel phones have this DirectTalk feature. My Nextel i880 has this feature.

To use it, you go into the menu to take the phone "off network". It reboots and then works in simplex mode to access the remote phone directly. It works the same as Nextel's Direct Connect feature so you get the little notification beep (although it sounds different) before the voice transmission.

Another nice feature is that you can select a channel / privacy code combination just like FRS radios to avoid unwanted traffic.

We used this in Yellowstone a couple of weeks ago and it worked quite well. We did not give it a through testing as it was used mostly for communications over open terrain approximately 500 - 600 yards away. In this application, it worked quite well.

Let me know if you need more info.

Craig.