Most airlines around Europe these days use alpha numeric call signs that bear no resemblance to the commercial flight number to reduce the chances of call sign confusion, so for example if XX5324 and XX5234 are scheduled to fly about the same time one might be 324F and the other 523Y or 52AQ or some other weird/whacky combination.
Some airlines are more keen on this than others, BA don't do it too often whereas easyjet or Lufthansa rarely use the commercial flight number.
S.