I would think, in one of two ways...change the destination recording used by the blacklist (which may affect other calls as well), or, forward the calls to a voicemail, or digital receptionist recording if possible (dummy extension?). I would word the recording something like... "Withheld number calls are not accepted, please unblock your number, and try your call again"
You could make an inbound rule:
Inbound Rules:
New CID rule.
name:
some callers.
caller id:
*anonymous*
route call to:
voicemail box for extension:
xxx
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