Typically, dependent upon temperature, the "crossover' altitude , at least in tropical regions occurs around 29000 feet.At this point in the atmosphere (the only point where the TAS for both mach number and the indicated airspeed is the same), your Boeing or Airbus or whatever aircraft, it doesnt matter which, will display a change from either IAS to MACH NUMBER, depending whether you are climbing or descending.
ATC like to refer to this point as the "transition" in their instructions, as you will "transition" from one speed reference to the other during, typically on descent, and if you stare at your FMS/FMC around this altitude you will spot the change over.......