Is it possible to set unconditional divert for some phone numbers/contacts, like on Android (like described here) ?
The only contact specific option is "Block", which is quite different.
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As of iOS 11.0 it is (still) not possible to selectively forward specific phone numbers to voice mail.
Well, let me point out call blocking applications like NoMoRobo. It can divert calls from a list of numbers to voicemail. The list it uses comes from outside the phone.
So it IS technically possible now, but there don't seem to be any applications that do it. This may be because no one has created one, or it may be because Apple doesn't allow use of those API's for that purpose. I wouldn't know.
But it is possible.
Edit: I spoke too soon, looking in the App Store, I see several apps that allow you add arbitrary numbers to a list, which will then be sent to voicemail. Search for 'call blocking list'.