Android

You should be able to do it from the native Google phone app - in the settings there's a menu for blocking numbers

I'm wondering about this too, and kind of vainly hoping for one that will prevent them from leaving voicemails.

Because that's how these spam callers advertise now.

I tried out my phone's built in spam blocking feature this past month but it only blocks the call, it'll keep it from ringing but these fuckers automatically leave a voicemail that in order to clear out of my inbox I have to be forced to listen to the first few seconds of "Hi! Did you know Comcast Xfinity has all the best blahblahblah..." before I can mash the number 7 to delete it. It's happened twice in my 1 month trial run of using the phone's built in defenses.

That's how they're advertising, through non consensual force. If they can't get you through the TV, or an internet ad, they're going to forcefeed it to you through your own voicemail.

I'll be damned if I'm going to be forcefed an ad.

So I went back to my usual practice of answering an unknown caller with silence. After 10 seconds of silence, I end the call. Keeps them from leaving ad voicemails. Fuckers can eat my entire ass and die.

I use a dialer called ACR Phone.

It does automatic call recording (poorly, like every other app that's on a non-root phone, but I digress) and caller ID based blocking. It also supports SIP accounts and other really useful stuff for me.

Yet another call blocker still works for me I believe. I have a blocked spam call from 2 days ago at least. It's the most simple one that I have used that works.