Cannot change Callsign

[PPLflorida]

Hello! I am very new in the Aerowinx World. I have the simulation since years, but just now i am really diving deep into it.
Last flight, i want to change my callsign in the Instructor panel. In the dropdown list i could choose speedbird. then in the field right next to it, there was already 47. I changed it to 87M. But immediatly after i entered it, it switched back to 47. Also the ATC called me Speedbird 47 the whole flight. How can i change that entry, so that i can choose my own callsign?

[Hardy Heinlin]

Hello, letters are not allowed. Just numbers.

[Jeroen Hoppenbrouwers]

AOC call signs (to be used towards company and customers) are numbers only.
ATC call signs (in Europe) may contain letters to help with reducing the amount of similar call signs. But this does not impact the AOC systems, as far as I know.