Actually disabling then reenabling "Nomorobo/Simultaneous Ring" (click it twice) opens a submenu where one is then able to disable "Nomorobo"- once saved the parent toggle (Nomorobo/Simultaneous Ring) reverts to "off".
The interface is somewhat counterintuitive and can be a frustrating time waster because simply trying to toggle "Nomorobo/Simultaneous Ring" off does nothing apart from instigating discussions like this one.
I believe the feature has prevented a good number of calls from ringing my phone but I suspect it has some unwanted effects on some needed automated calls.