regor,
RT-callsigns aren't just introduced out of the blue. Each operator that uses flight numbers is assigned a unique three-letter ident that is registered with ICAO through the operator's national authority. The operator has the option to phonetically spell the callsign (with a few exceptions for airlines that have been around a long time such as KLM) or request an RT-callsign, such as Speedbird, Reach, Clipper or - in our case - ferry.
A list of three-letter idents, RT-callsigns and name and address of the operator is kept in ICAO Doc 8585.
I know that at least the FAA recommend to put a remark in the flightplan for several months to get the RT-callsign known to all agencies.
Erik.