Rich - Indeed, the last four digits remain the same in the UID. It's the first three that change and I hear the trooper's ID whenever he calls the dispatcher, so I know it's the same car/trooper. I'm not so interested in whether it's the same person each time or not, as I am in identifying the car number.
This same scenario happens with my local sheriff's department. I hear them identify when calling, and sometimes their first three numbers are different, like i700xxxx or i710xxxx, where the last digits remain the same.
The mobile extending ability has to be the reason for the change in UIDs. By listening carefully, and visually checking the UID in the display, I can usually distinguish between their usage of the car radio or portable, thus allowing me to alpha-tag the UIDs correctly. Only problem is, I have two UIDs for every instance of an ISP car number (ie., car & portable). (It's really not that bad, I guess...)
Thanks so much for your reply and information. I've been learning a lot since I got my BCD996XT & 396XT. It confirms my thinking of what was going on with the different UIDs appearing.