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

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

CALL can pass back values to its caller using parameters that are
declared as OUT or INOUT parameters. It also "returns" the number of
rows affected, which a client program can obtain at the SQL level by
calling the ROW_COUNT() function and from C by calling the
mysql_affected_rows() C API function.

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

URL: http://dev.mysql.com/doc/refman/5.0/en/call.html

