On the Treo600 after I ticked the option in Preferences/ keyguard / on a call box. The result was I found it impossible to terminate a call (it disables the screen box) I ended up pulling the SIM card out. The manual has no entry for terminating a call.
Thanks to Jay Tamboli on the Visacentral user group
From page 47 of the CDMA manual:
Do one of the following:
Choose Hang Up All [ you can do this with the five-way ]
Press the headset button (if the headset is attached)