 In order to reduce the possibility of two callsigns on one frequency at any time sounding too similar, a number of airlines, particularly in Europe, have started using alphanumeric callsigns that are not based on flight numbers (i.e.DLH23LG, spoken as Lufthansa-two-three-lima-golf, to prevent confusion between incoming DLH23 and outgoing DLH24 in the same frequency).Additionally, it is the right of the air traffic controller to change the 'audio' callsign for the period the flight is in his sector if there is a risk of confusion, usually choosing the tail number instead.