While you can use the hotline in private, you can't object to any matter in public.

From what I can tell, there are many issues that aren’t off limits to criticize on Chinese social media. In fact, recurring social media complaints are what spurred development of the hotline system.

It’s mainly complaints that are considered sensitive or destabilizing that are suppressed. This should sound familiar to those of us in the West. Germany actually goes farther by directly funding left-wing protest groups, as these are not considered destabilizing.