Unlike broadcast radio stations, licence call signs are issued randomly by the FCC. The call sign for my fire deparment here in New York is KJD-401. I don't belive the letters have any meaning, other than most licences will start with a K or a W. It seems that 3 letters indicate base stations, 4 letters in a call sign are for mobile licences. I don't claim to be any expert, but i've been around this stuff to figure it out on my own. I hope this answers your question.