Actually if use device lock, it will lock the screen as soon as phone goes to sleep. So if you are on a call, the device will and should lock b4 the call is ended. I have currently been using slide lock and I am getting the same results. Both items can be found at xda forums.
http://forum.xda-developers.com/show...ht=device+lock
http://forum.xda-developers.com/show...devicelock.cab
the first thread is for slide and the second is for device lock. Colonel had these in his customised roms and they were great apps for me.