SpamBlocker on fdroid can block any numbers not saved to your address book. It can even make exceptions for any repeated numbers that aren’t in your contacts but call multiple times within some time period (5min, 30min, whatever). And it can temporarily whitelist a number you dialed recently in case you’re expecting a call-back.

It also has the ability to download spam phone number databases from the internet if you want, but to be honest, I tried setting up that feature and couldn’t figure it out. So while it can use a 3rd party, it isn’t required.

I can’t speak for it as I’ve never used it (I don’t have any spam call issues, fortunately) but SpamBlocker sounds like it would fit the bill. It’s FOSS, available on F-Droid.

They do have online features that contact APIs, but they release a build without internet privileges if you so desire.

I use Carrion, you can download a database of high confidence spam numbers and then take the internet permission away if you’re on GrapheneOS to ensure there’s nothing being transmitted. It also enforces stir/shaken.

github.com/x13a/Silence

Available on fdroid. I’ve never checked to see if it phines home tho