I have a "spam" contact which is set to "route to voicemail" right away. (See How to gracefully block calls for an example)
This setting frequently turns off on its own, so calls go through anyway.
Why would that be the case? What can I do about it? I'm using my carriers (Three/Hutchison 3G) normal voicemail service.
Nexus 5X, stock android 7.1.2.