Hey guys I'm new around here, but an iOS developer, and I used to work at Apple too. I had this exact same issue and I tried all the different apps - NomoRobo, Hiya, etc - and the best one BY FAR is RoboKiller.

I think it's like $10/yr but it's so worth it. I get max 1-2 calls per month now vs 3-5 per day. They also have a fun feature that auto-answers the spammers phone calls and messes with them via pre recorded audio, that you can listen to.😆 There's a free trial, so try it out. I can't post URL's yet but google RoboKiller.