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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:

  1. Go to “Settings”“Blacklisted Numbers”.
  2. Click on the “Add” button.

  1. In the “Blacklist” window:
  • Set the “Incoming caller ID to be blocked” field to the value “Anonymous” or “anonymous”. In this case, we will set the field to “*nonymous” to match both values (The * will act as a wildcard).
  • 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:

v: SIP/2.0/UDP;branch=z9hG4bK-8b2f242058b3802ce60c00f1db2b8201
f: "Anonymous" <sip:anonymous@>;tag=3727603634-668969
t: <>
Max-Forwards: 13
m: <sip:d499d4130a8ff5ed6ed0238096db60f6@;transport=udp>
Proxy-Require: privacy
c: application/sdp
l: 268

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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