I need an app that sends all calls not in contacts directly to voicemail

I don't but I would gladly install such an app. I swipe all unknown callers to the abyss now with only a rare few bothering to leave voicemail. Such an app would work well for me.

 

Most phones can do this with the default phone app, kind of. You just need the option to customize ringtones for each contact which most phones have. Non contact numbers will be silent but still "ring" for 15 seconds.

Not quite as good as having them go straight to voicemail, but if you can't find another app to do that at least you won't hear the phone ring.

1. Make your default ringtone an audio file that is silent.
2. Edit all your contacts and change their ringtone to your normal ringtone.
3. Some phone apps let you create groups of contacts and assign a ringtone for that group. This might be faster if you have lots of contacts.

Archive.org has a silent ringtone if you need one:
https://archive.org/download/SilentRingtone/silence_64kb.mp3

Location for ringtone audio files on your phone:
/sdcard/media/audio/ringtones/