When you want to call stored function (and want to read its result) which executes DML queries (insert, update, delete) inside its body you can't use "select your_stored_function(:param1, :param2) from dual" because you will receive "ORA-14551: cannot perform a DML operation inside a query" error.
In order to call such function and get its result you need to wrap it into nested procedure with OUT parameter like this:
DECLARE
PROCEDURE caller(return_value OUT NUMBER) AS
BEGIN
return_value := your_stored_function(:param1, :param2);
END;
BEGIN
caller(:return_value);
END;
and bind to :return_value variable to get the result of function.
