I'm using AT&T's Call Protect service in addition to the trial month of Nomorobo to good effect. I've encountered just a little weirdness with AT&T's service wherein it seems like the service doesn't block the incoming fraud call in time or blocks it after it's rung my phone once or maybe twice. Other times it's worked fine. I'm not sure if some of this has to do with my frequent use of AT&T's Wi-Fi Calling, VOIP service on my devices.
As far as Nomorobo, it appears to basically work as advertised, and it adds a simple red dot on incoming robo calls via CallKit. This is good because I think AT&T's Call Protect is geared more toward preventing fraudulent or scam calls, while Nomorobo is geared more toward blocking robo calls or marketing and spam calls.