Does Isolation Level Serializable avoid All concurrency problems?

Question

In Oracle I ran a query in PL/SQL Developer like this :

declare a number;
  begin 
    EXECUTE IMMEDIATE 'ALTER SESSION SET ISOLATION_LEVEL= SERIALIZABLE';
    select count(*) into a  from item w where w.Item_num='MOH601' ; 
    dbms_output.put_line(a);
  end ; 

I don't commit or rollback this session , in another session I ran this :

begin
  EXECUTE IMMEDIATE 'ALTER SESSION SET ISOLATION_LEVEL =SERIALIZABLE';
  insert into item w
    (id, item_num, )
  values
    (200, 'MOH601');
end;

As I known serializable doesn't let phantom read , but I can commit this .