Why is this query not using indexes?

SQL> SELECT * FROM (SELECT dsl_modems.pk_dsl_modem_id, dsl_modems.serial_number, mtm_dsl_modem_2_cus tomer
  2  .fk_dslam_port_id, mtm_dsl_modem_2_customer.fk_customer_id
  3  FROM dsl_modems, mtm_dsl_modem_2_customer  
  4  WHERE      mtm_dsl_modem_2_customer.fk_dsl_modem_id (+)= dsl_modems.pk_dsl_modem_id
  5  AND      dsl_modems.fk_modem_status_id = 'WORKIN'
  6  AND fk_dslam_port_id IS NULL 
  7  AND dsl_modems.brand='ST716g'
  8  AND (fk_customer_id IS NULL
  9  OR   (fk_customer_id IS NOT NULL
 10  AND   pk_dsl_modem_id
 11  NOT IN (SELECT fk_dsl_modem_id  FROM mtm_dsl_modem_2_customer