Out of interest, who decides what the alpha numeric callsign will be?
Is it the airlines own system?
Airlines are free to choose their own alphanumerics. Some, Lufthansa for example, appear to have a system that does it for them. Others, like BA, look like they have a bloke who sticks his finger in the air and makes one up (with the exception of SHT alphanumerics, which do follow a discernable pattern).