Syntax:
CALL sp_name([parameter[,...]])
CALL sp_name[()]

The CALL statement invokes a stored procedure that was defined
previously with CREATE PROCEDURE.

As of MySQL 5.1.13, stored procedures that take no arguments can be
invoked without parentheses. That is, CALL p() and CALL p are
equivalent.

CALL can pass back values to its caller using parameters that are
declared as OUT or INOUT parameters. When the procedure returns, a
client program can also obtain the number of rows affected for the
final statement executed within the routine: At the SQL level, call the
ROW_COUNT() function; from the C API, call the mysql_affected_rows()
function.

URL: http://dev.mysql.com/doc/refman/5.1/en/call.html

