Blocking voicemails is not available in Truecaller as It requires a "Pick up, hang up" method and the Android system does not allow third-party apps to automatically pick up calls. We only use "hang up" to block calls which makes it possible for the person to leave a voicemail.
On the side note, the "pick up, hang up" setting could also backfire. A good example would be situations where scammers charge instantly as soon you pick up the call, therefore, it's safer to just hang up.
Additional information: When you hang up a call by tapping "red phone" the other end will either hear a busy tone or "person not reached" (depending on carrier) and by default, the TAD (telephone answering device) will kick in and ask to leave a message. But if you pick up and hang up, TAD won't kick in.
Thank you for your understanding.