I know this is two years old but I’ve been searching for a fix for six months but going to post in case this helps anyone at all. I had the same issue and had all the normal stuff off. DND, silence unknown, double checked I didn’t block people, etc. Problem that I had was settings-phone-calls on other devices. I have a iPad and Mac mini I allow to use my phone. Some setting in my mac mini was causing the decline of calls. I haven’t figured out exactly what yet and will continue to delve into it. I don’t think my Mac is up to date so that’s a possibility. When I deselected Mac Mini all my calls returned to normal. Hope that helps.