How to Check for and Block “Anonymous” Callers

If you want to block calls from “anonymous” callers, you can leverage the “CallerID Blacklist” feature. From the 3CX Management Console:

  • Go to “Settings” > “Blacklisted Numbers”.
  • Click on the “Add” button.

Blackisted Numbers Sample

  • In the “Blacklist” window:
    • Set the “Incoming caller ID to be blocked” field to the value “Anonymous” or “anonymous”. In this case, I have set the field to “*nonymous” to match both values.
    • Set the “Description” field for informational purposes.

At this stage, you can check an inbound call and you should see that the anonymous call is rejected correctly.

This is a typical INVITE from a caller whose CallerID is hidden:

INVITE sip:20112011@ SIP/2.0
Via: SIP/2.0/UDP;branch=z9hG4bK7fbdc85f;rport=5060
Max-Forwards: 70
Contact: <sip:Anonymous@>
To: <sip:20112011@>
From: “Anonymous”<sip:Anonymous@>;tag=as4f07a8c6
Call-ID: 5ab5219836fe3f983650927f1c785544@
CSeq: 102 INVITE

Sometimes your telecom provider will deliver the call with “unknown” or some other text instead of “anonymous” – simply examine the incoming INVITE particularly the “From” field. You can then add or adjust your Blacklist rule accordingly.

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  1. Ron Block

    Is there any way to play a message to the call before terminating the call. Such a message might be “We do not accept … calls.”

    August 27, 2011 at 8:40 pm
  2. I’ve had some hackers try to get into my office 3CX system, from the Czech Republic, to make long distance calls. The caller ID on these has been “asterisk”. Is there any reason why I should not use this same method of blacklisting “anonymous” caller ID’s, to blacklist “asterisk” caller ID’s?

    August 29, 2011 at 9:10 am
    • Kevin

      You can use this method to blacklist such CallerIDs, but note that this is not intended as an anti-hacking tool.

      If you want to harden your system against hackers, you will still need to take advantage of the REAL anti-hacking features provided by 3CX:

      * The “Anti-Hacking” tab inside “Settings->Advanced”
      * The “IP Blacklisted” tab inside “Settings->Advanced”
      * The “Extension Options” section inside the “Other” tab for each extension

      This article will also give you some insight about securing 3CX Phone System:

      August 29, 2011 at 6:51 pm
  3. Kevin


    To achieve this, rather than using the Blacklist concept, you could create an Inbound Route that is Caller-ID-based from the menu item “Add->Did / Inbound route”, and route such calls to a Digital Receptionist with the appropriate message, no digit-based actions, and a timeout of “1” second.

    August 29, 2011 at 8:46 pm