I rather like the wizzair callsigns...
eg. wzz201c
the 2 is then the actual flight number
the 0 is an unknown to me, but i guess it has something to do with the destination.
the 1 shows it's an outbound flight (mostly out of poland)
and the c is added to get the alphanumeric element in there as well, and goes up with the flight number
So, outbound flights: 101b, 201c, 301d, 401e, 501f
and the returns are then 102b, 202c, 302d, ...
At least those are some of the flights we get through the sector almost every day.
(i must admit wzz771z looks a bit confusing sometimes)