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How to block incoming faxes based on sender number/TSI



This Article is under Review. It was published for an earlier version of KCS, but it may no longer be valid for the current version.


Not only emails may contain spam messages, also faxes may be unwanted. Therefore some customers want to block the reception of such known senders directly, while others may want to route it to a NULL TUM channel, effectively deleting it while producing costs for the sender. Here we have a look at possible ways to block the reception entirely. What basically needs to be done is tell TCOSS that the originator is invalid. On a technical level, we create a "bad originator" error in TCOSS so that the send order cannot be created.

This can be done via the following way:

In a default environment, FAX is the originator service for inbound faxes. This is defined in the config line 235, position 3.

Now, you need to define the caller ID that needs to be blocked in the ARR99, INBOUND section

For example, if you want to block faxes from the number 1234, you would need to add this:

F:1234,invalid, user "invalid" does not exist -> TCOSS will not accept the message


  • it is quite simple to add new sendes to this blacklist.


  • In the other routing sections the F:1234 MUST not be changed, or at least when entering the inbound section must look in that way
  • If a user tries to send now to: FAX,1234 this will also not work.
  • If the sender does not send its own DID the TSI of the sender is used, which can be faked, but can still be entered here if known.

As an addition to the above entry, you can also add the numbers to the config lines 254 up to 283 like this:

In this example, the numbers 1234 and 5678 should be blocked.

Enter somewere:


Then, in the RR99 you add this instead of the entries from further up:

In rr99:

F:0001,invalid, user "invalid" does not exist -> TCOSS will not accept the message

if the caller number is 1234 or 5678 it is routed to 0001, in RR99 refused.


  • It is possible to send to FAX,1234 and FAX,5678


  • If sender does not send DID, you would have to enter the TSI instead.
  • Every time you enter a new number, you must restart the node of these channels
  • There is only a limited number of lines available in the number conversion table
  • The first match wins; if you have something like "3~=~" before this, it will not work

If you block faxes in that way, the sender will get a fax interruption during the initial training phase of the fax. This means, the initial call is accepted and as soon as any data is to be recieved, TCOSS will close the connection again. The sender will not get a Sent OK.

Applies to

  • Tested with KCS 9.1 and up
  • ISDN Fax and FoIP

Keywords: Spam, Fax, invalid, block, bad originator