Hi Kirk.As has already been said,the airlines choose the callsign. Alphanumeric ones tend to stop similar callsigns by the same company however if diffirent companies use the same letters at the end of the callsign then the problem remains. BMA and BRT have callsigns that end in CW that arrive at the same time in the scottish tma. ATC can also request a change of callsign for a portion or for all of the flight for callsigns sounding similar or looking similar on radar eg SHT6E SHT6F
cheers,rolaaand