If your Call Blocking setting is set to Block Calls, the blocked caller hears nothing as they are immediately disconnected.
If your Call Blocking setting is set to Sends Calls to Voicemail, the blocked caller will be able to reach your voicemail box. No notification is given to them that they have been blocked.
You can manage this setting by navigating to the Advanced Settings page.
Call Blocking Pick-Up, Hang-up Method Enabled:
Call Blocking send to Voicemail Method Enabled: