Sending to voicemail is only available to trial or subscribing customers. When you receive a nuisance call, please report that call so we can protect other users.
Also, make sure you set your Trust Score to a low level (see - How can I adjust the Trust Score?), and you have accepted that category of call in custom settings (see - How do I use custom settings?).
If a nuisance call did not have a call category assigned, please add it at the bottom of the Lookup tab. By adding the category, we can then let others know when they receive calls from that number.