It seems to go in both directions: you can ask for one that has some meaning to you (which you can then potentially get if no one is using it), or you can try to make up a backronym. I don't remember good backronym examples but I know they exist. The stations will also vary in whether they prefer for people to pronounce the whole callsign, or just part of it, or spell it out. (Technically I think the callsign is always spelled out when used as a callsign, but not necessarily when used as a brand.)