Decline a phone call and skip voicemail?

How do I decline a call from an unrecognized number and also not let them leave a voicemail? I get spam calls from new numbers daily and I don't want them to validate my number by getting my voicemail. To avoid connecting to my voicemail I have been answering and quickly hanging up. Is there a better way? I believe hitting the volume button once, or the side button ignores the caller, but it sends them to voicemail. I don't want the caller to get to my outgoing message. I know I can only accept calls from my contacts, but I don't want to do this as I have other new calls not in my contacts that I know are legit. And because these are always new numbers, blocking doesn't work. I know they are spam when they are coming from another country half way around the world and I want to stop them getting to my voicemail.