There are three ways to define the call-sign on a flight:
N-OC will determine the Call-sign in the following order (1 has highest prio)
1. Entering a callsign in the flight-plan tab.
2. Call-sign "rule" in the Call-sign table (under menu: Schedule).
3. Reading the ICAO code from the Prefix of the flight number.
How to use the Call-sign dialog:
In this dialog it is possible to translate an IATA flight number to a Call-sign. Enter Prefix, Number and Valid From/To, suffix is optional. This Callsign will then be used for incoming Euro-control messages. It's also displayed in the Leg tool-tip and Activity Info (please note the order above).