I enabled the Silence Unknown Callers feature [0] in iOS and haven’t answered a spam call since. The downside is that it sends every call from unknown numbers to voicemail, including legitimate ones. It’s a worthwhile trade-off. [0] https://support.apple.com/en-us/HT207099
I tried it, and within a week I missed an important call. I was waiting by the phone, and it just never arrived. I realized too late that it had been silenced. It's very frustrating that we can't just get something like a spam filter, rather than a blanket "silence everything unknown."
It would kind of solve the problem if a norm emerged that these types of companies first send a confirmation email with the phone number they'll be calling from so you can add it to your caller id. But it's a lot of extra work for people and I think it's probably asking too much of the average non technical user.
Even better, why couldn't we have something like SSL certificates or DKIM for phone calls? People for the most part understand the lock icon and a verified flag in a user interface. Then a call could be signed to know that it's coming from a particular entity.
It seems like some other countries have solved this problem by moving away from regular sms and phone calls and instead letting a private company own all communication, like WeChat in China. Which obviously is quite problematic in other ways, but honestly at this point that would be an improvement in my opinion, if businesses started only contacting me through Facebook Messenger or Whatsapp so I could see who every message is associated with.