The boatswain's pipe is the 'modern day' descendant of the flutes used by the Ancient Greeks and Romans to convey orders to the oarsmen and galley slaves. The first use of the whistle in the English Navy was during the crusades of the 13th century.
The 'Call' (or 'Pipe') is named after the Boatswain. He was the officer who, being in charge of the rigging, issued more orders than the other officers. Frequently, the human voice was not sufficient to carry orders to the men climbing the rigging, especially in storm conditions. The high pitch of the Boatswain's 'Call' could be relied upon to be heard above even the worst howling gales.
The Boatswain's Pipe
Four pages of info (with images), including how to hold the pipe, "Piping Orders", etc.