Keith is correct, this dialog triggering functionality is a value-add, unique to iCab.
I'm working from memory, but here goes: On the original Railsounds products featuring intelligible crew/tower dialog (probably RS5 or RS5.5), there was a slightly different sequence of aux-key, 2 key and 7 key button presses. In that early version, the engineer-first/clear inbound was triggered by aux1, 2 in motion. But a change was introduced for Legacy and that particular phrase was removed from the cab trigger sequence.
However, the underlying "am I clear inbound?" speech and its corresponding direct dialog trigger command was preserved in all subsequent legacy locomotives specifically to enable future products like iCab. While it is fun to think of these as "hidden" dialog features, they're in plain sight should you review the public Legacy command protocol document.
During iCab development, we added new buttons to allow triggering that phrase, plus a few others, like "say current speed" and "say fuel level."
The iCab app offers an improved dialog experience, as it is not constrained by a lack of unused buttons, and iCab dialog buttons can be given clear and descriptive names.