Chuboy - there was a time when the last two letters of the ICAO destination are used - eg speedbird 76AT flying to Athens, LGAT, or some other indicator of destination. I well recall operating the 'Speedbird xxWW' on flights to LOWW, Vienna, which was known to Maastricht as 'Speedbird xx double scotch' on occasions.
All well and good for our beleagured UK controllers until all flights inbound to XX arrive near destination with the same suffix. It would be like having all movements into LHR suffixed 'LL'
You will also sometimes see xxxxP for a positioning flight or xxxQ for a training flight and a few other suffixes - and sometimes, of course, there is just NO logic in the callsign at all, with sometimes a single letter, sometimes a double, at the end, which is the best way to try to eliminate confusion.
The CAA published a thorough paper on callsign confusion in the late 90's which is worth reading.
www.caa.co.uk/docs/33/CAP704.pdf