See Honeywell document: I am afraid you'll have to change your ATC call-sign in your RMU each time. Maybe it has to do with your type of transponder.
http://www51.honeywell.com/aero/comm...n_Mandates.pdf
"Important Note about ID: The MST 67A has a feature that allows it to automatically determine the aircraft Registration Number (Tail Number) of US registered (N numbered) aircraft. If the aircraft is US Registered and operated where the tail number is used as the call sign (Flight ID),
then the operator does not have to manually enter the Flight ID – therefore a new Flight ID control head is not required. The transponder automatically reads the Mode S address assigned to the aircraft.
If it falls within the range of addresses assigned to the US by ICAO, it will automatically convert that Mode S address to the tail number of the aircraft and place it in the Flight ID register for compliance with Elementary surveillance.
If the aircraft is operated where the Flight ID is not the tail number or the aircraft is not US registered,
then a Flight ID control head is required to allow for manual entry of Flight ID."