I think the two of you are getting confused here.
The ICAO code is BOX - as used for flight planning purposes and which will show up on flight tracking programmes, flightradar24 and the like.
Any printed schedules will be prefixed with 3S.
I can personally attest that their r/t callsign as used in communication with ATC is German Cargo. Any flights done for LH Cargo would presumably use their r/t callsign of Lufthansa Cargo.
A similar analogy is BA
ICAO BAW
IATA BA
R/T callsign Speedbird