The individual number you are refering to is the ICAO address and is 24bits long, this is linked to the registration of the aircraft and is broadcast from the aircraft.
Callsigns are set by the crew as Heathrow Director points out, however I believe these are broadcast from the aircraft not looked up by an ATC computer, hense the silly ones seen!!
I am not aware of the aircraft type being broadcast, yet the info appears on the screen, so i assume it is obtained from the ATC computer and is found using the ICAO address? Is this so?
smithgd