International call sign prefixes are defined by the International Tele-
communications Union (Palais Wilson, Geneva, Switzerland) and are published
in many places. Look at various publications of the American Radio Relay
League (ARRL), e.g., their Amateur Radio Handbook, or in various Shortwave
Radio Directories. Also, reference works such as "Reference Data for Radio
Engineers."
It's a pretty haphazard system, I'd say - worse even than telephone numbering.
The block HCA-HDZ is Ecuador, while HBA-HBZ and HEA-HEZ are Switzerland.
The US has all the W's, K's, N's, and half of the A's. The third world
fights for scraps of address space.
Martin Ewing, AA6E
Caltech Radio Astronomy