The chances of other people knowing that you have blocked them can increase over time since if the person you have blocked is someone who knows you closely and calls repeatedly, there is a chance that he might get doubts about you blocking them. Since every time they call you the call is redirected towards the voicemail, so it might be doubtful. If you have blocked an unknown user, they might never know that you have blocked them, since they won’t call you repeatedly to check on you. Since the mechanisms are the same for everyone, there is definitely a chance that the other person is aware of it, but if someone is bothering you, then you definitely need to block them, irrespective of the circumstances.