As mentioned above, pressing Decline works well. It’s no issue to block the number.
Unfortunately "decline" doesn't always do the job. It sends the caller to voicemail (or to a busy signal, depending on your carrier settings), which is not desirable because then they would know that you declined.
If I'm watching a movie and I decline, the caller knows that I, well, declined. And tries again. And again. Then I come across as rude.
If I could just swipe the banner away and let it ring through, the caller would remain with the doubt that maybe I just didn't hear the phone, or whatever. I guess it's a cultural thing.
My iPhone has the word "phone" in it, but I don't use it as a phone really. For me it's a portable computer which happens to also be able to make phone calls. I think I will migrate to a data-only SIM at some point. That's what I already kind of do today: I divert my phone number to a VoIP account which then "rings" in the Acrobits Groundwire app (with Callkit disabled). There I can just swipe the banner away and I'm good. It's an ugly workaround, but it works.