Similar idents
I am under the impression that in Europe airline idents are composed of numbers and letters, such as BA24R or Kestrel 39WX, in order to solve the similar sounding ident issue. That identifier is often not the flight number used in the airline schedule but just externally to ATC to alleviate confusing similar call signs (usually 4 digit similar numbers).
Would someone please clarify this and if it is the case, then which agency assigns that call sign?