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@172.18.1.6:5060 SIP/2.0
Via: SIP/2.0/UDP 62.62.62.3:5060;branch=z9hG4bK7fbdc85f;rport=5060
Max-Forwards: 70
Contact: <sip:Anonymous@62.62.62.3>
To: <sip:20112011@172.18.1.6:5060>
From: “Anonymous”<sip:Anonymous@62.62.62.3>;tag=as4f07a8c6
Call-ID: 5ab5219836fe3f983650927f1c785544@62.62.62.3
CSeq: 102 INVITE
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, SUBSCRIBE, NOTIFY

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.