Anonymous Call Filter (ACF) is part of the Intelligent Call Diversion feature. When it is activated, all incoming calls from anonymous/blocked numbers will be routed to your pre-set second destination (e.g. voicemail).
To turn anonymous call filter on or off, logon to the Reroute control panel and simply select 'Anonymous'. You will then see confirmation text beneath the panel, stating "OK! Your request has been accepted."
At the same time, you can also change your second destination (e.g. from landline to voicemail) to make sure the unidentified calls go to the right place.
