select 
  coalesce(A.tel,t2.tel) tel,
  A.operator operator
from
-- телефоны 'длиннокодовыХ' операторов
  (select * from tel_codes c left join m_tel t on c.code=substr(t.tel, 1 , 4 ) ) A 
-- телефоны 'первичных' операторов
  left join m_tel t2 
	on A.tel is null and A.code=substr(t.tel, 1 , 3 );
--ЗЫ: схему данных менять надо, однако.

select t.tel
  , coalesce(
    (select operator from tel_codes where code=substr(t.tel, 1 , 4 )),
    (select operator from tel_codes where code=substr(t.tel, 1 , 3 ))
  ) operator
from m_tel t;