So, it refers to "telefone fixo", right

  Yep, I think so.

  These #s are the ones that you would call in case the PABX system is down, let´s say, if the PABX system is burnt by a short circuit or something like that.

  In other words, with these numbers you usually would call and inform the "ramal'' (certainly Dale, Teacher, Milton or other buddy will translate "ramal" to us), but in case the system is out you will have only the these given numbers.  My thinking, but I am curious too, and craving for a confirmation.    ;-)