Oracle 18c

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create table t (
  cta_que_factura number(6),
  num_telefono number(12)
);
insert into t (cta_que_factura, num_telefono) values (123, 666666666);
1 rows affected

insert into t (cta_que_factura, num_telefono) values (456, 0);
1 rows affected

insert into t (cta_que_factura, num_telefono) values (456, 777777777);
1 rows affected

insert into t (cta_que_factura, num_telefono) values (789, 0);
1 rows affected

select * from t;


CTA_QUE_FACTURA
NUM_TELEFONO


123
666666666

456
0

456
777777777

789
0


select *
from (
  select t.*,
    row_number() over(partition by cta_que_factura order by num_telefono desc) as rn
  from t
) x
where rn = 1


CTA_QUE_FACTURA
NUM_TELEFONO
RN


123
666666666
1

456
777777777
1

789
0
1