In Graphene’s phone app, you can go to Settings -> Blocked Numbers, and enable “Numbers not in Contacts”. That will send any call from numbers you don’t have saved directly to voicemail. I still get the occasional voicemail message that’s just 2 seconds of silence (they didn’t hang up in time) but it stopped all spam calls for me.
In Graphene’s phone app, you can go to Settings -> Blocked Numbers, and enable “Numbers not in Contacts”. That will send any call from numbers you don’t have saved directly to voicemail. I still get the occasional voicemail message that’s just 2 seconds of silence (they didn’t hang up in time) but it stopped all spam calls for me.