I read this, thought “that’s gotta be something you can do in Termux”, searched it, and sure enough; that’s something you can do in Termux.

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.