Lufthansa is adopting a new alphanumeric callsign system that it hopes will reduce confusion by making callsigns easier to differentiate phonetically. Many airlines and air traffic control units have experienced confusion when two subsequent flight numbers like 7566 and 7567 are in the same sector, so Lufthansa says it intends to use a wider variety of three unit callsigns mixing letters and numerals, for example 5CX (five charlie x-ray).
Source: Flight International