Add DTMF handling for '*' (repeat menu) and '#' (return to menu/exit)

Adds support for the following DTMF buttons:

'*' - repeats the last menu.
'#' - exits from the change folders prompt. If already in the main menu, plays bye and hangs up.
Most of this is still duplicated from the code that is run during the intro. There should be a way to move the duplicated code so that we don't have to reproduce it twice.