Oh. I did not realize that flag did that. You have bested me!
That's a good question. I don't know what controls it, although i suspect it's hardcoded to list only cars in the engine. I once tried to change that, but the cell phone code just gets the list from the engine, and then tells the engine which one you selected by index. It can't change what the options are. But, i may be misremembering something else, because i can't remember right now -where- that code is.
edit: Right, here it is, in the cell_phone.lua file, under the heading "-- GARAGE".
It makes an internal function call (the kind of code we can't change) to cell_phone_pad_populate. When you make a choice, then, it just calls another internal function, cellphone_choose_vehicle(data.id), with data.id being the ID of the vehicle in the list returned by the first function.
Maybe i'll play with it a little more. I want to see what ID's it's using, and whether they can be altered. I don't remember what happened last time i played with it. But, even if they could be altered, it'd be another trick entirely to change the list displayed in the phone.