Unknown calls that are coming through is a sign that there is no Caller ID available to display/detect.
Anonymous Call Rejection only works when the customer has knowingly/ actively blocked their Caller ID information from coming through.
Unfortunately, this is not a problem specific to Comcast, but also an occurance with other telephone providers as most National Telephone providers use a National Database of registered telephone numbers to display Caller IDs.
If there is no Caller ID information to display it will still come through, however be displayed a UNKNOWN or UNAVAILABLE
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