I think the question has been answered pretty well, but as the designer of the original legacy dialog behavior, I'll chime in. Dialog evolved over time, and since my time at Lionel ended almost 10 years ago, I may not be up to date on any changes since then.
For original TMCC, the numeric <2> played a burst of unintelligible radio chatter, which was preceded by a 2-3 second delay. There wasn't enough sound storage for more. There may have been some special case engines or rolling stock that had a unique piece of themed dialog, but as was stated earlier, you won't learn about this in the control device manual, it would be unique to the engine or rolling stock.
Why the delay? It was Neil's idea to have talking caboose as part of the <train>. Pressing the <2> immediately played the caboose, and 2.5 seconds later the locomotive "answered." If you did not have the caboose, you were stuck with the 2.5 second delay anyway.
Fairly early, the <7> was introduced as the voice of the tower. Originally this was probably just a single phrase giving the train clearance to depart.
At some point, the rail sounds hardware and memory storage were increased. Other train sound manufacturer's systems included intelligible dialog, but with the still limited storage in general, there was a lot of repetitive phrases. So Lionel wanted to stop using the static burst and put in understandable dialog.
I wanted dialog to tell a simple story, to add play value The story was :
- not going yet
- ready to go
- wow, just started going
- still going
- almost there
- have arrived
The Legacy manuals describe this story arc. It is conditioned on the presence or absence of the aux key as well as the motion state of the engine (including the state "recently moving")
The choice of <7> vs <2> key was used to determine of the tower spoke first or the engineer. Repeated denial of clearance by the tower sometimes caused a crabby/sarcastic acknowledgement by the engineer.
I could go on and on about this, but I'll leave it there for now. Yes, within a single part of the story there is no way to predict if the voice is "Roger that, clear inbound" or "copy that dispatcher, pulling in now." Sorry!