Actually, DIAD 2 did get messages. Not at the beginning of course.
DIAD 2 didn't have a radio built in like DIAD 3 and 4. With the DIAD 2, you had to put it into a vehicle holder (DVA). The DVA had a cell radi connected that could allow the DIAD 2 to communicate with the mainframe.
The DIAD 2 would send up tracking information and then download messages. It took much longer for a message to be sent and received.
If you remember, in order to make that work, UPS had to create a consortium of many, many cell phone companies.
That was quite a feat back in the day.
P-Man