Hello Bomber
TC ATCO and I'm certain our callsigns originate from your filed flight plan rather than from your Transponder. For example a/c with Elementary Mode S or no Mode S at all still appear in our stack lists with their c/s.
So as Over & Out said, callsign from flight plan, all other data from your downlinked parameters.
As a slight aside I am really quite concerned about these SBS devices - potential for rather undesireable uses...