Let's make things clear:
Unknown Number ON: performs a phonebook check, so calls or sms's from numbers wich are in phonebook are accepted.
Restrict Number ON: performs call block if the number can't be identified, eg, it's a hidden ID number (restricted).
Quote:
Originally Posted by mcdj
The new update fixes the slow startup. Good work.
I set my blacklist to one number, 000-000-0000, and set it to give a busy signal. Today I got a voicemail notification from 000-000-0000! 
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In this case, i see one possibility. The call didn't reach you iPhone, so it didn't effectively sent a busy signal to that number. It can happen. Is like the carrier was so busy it could not find your phone, and then it redirects the caller to your voicemail.
Try to turn iphone off and call it. I think you shall be redirected to voicemail.