Skip to main content

How to enable the verification of the CSI before sending a fax




In rare cases it can happen that a fax is sent to an incorrect receiver – different “Called Station Identifier (CSI)” response – as a result of a telecom redirect. To guarantee that the fax reaches the correct receiver, the CSI should be verified before actually sending out the fax. The verification for sending to the correct receiving fax machine, can be achieved by specifying the so called answer- back.


There are 2 possibilities how the answer-back can be configured:

Possibility 1

With the 'TCfW-Communication Server Client' in a separate field of a fax address, as shown in the below screenshot. Go to 'Admin' -> 'Address book...', choose 'Recipients' and create a new Recipient.

Screen Shot 2018-07-20 at 10.09.08 AM.png

Note: This has to be done for all fax-recipients as it verifies that the returned CSI matches the answer-back for this recipient.

Possibility 2

The answer-back can be entered when sending from any LINKs.


  • TC/Link-FI with the parameter “AB=<answerbackstring>
  • TC/Link-SM by adding the answer-back in the address: “[fullname#][service#]number[#answerback]@mail-domain”.

How does it affect the message flow

When sending to a configured number with answer-back (as described in 'Possibility 1') or adding the answer-back to the recipient address (as in 'Possibility 2'), it will be checked if the specified answer-back string is a sub-string of the CSI-string (received from the distant fax machine).

If this - answer-back matches the CSI - is the case, the message will be sent out.
Otherwise – if it doesn’t match – an error will be returned, stating “XK – answerback mismatch”.

Applies to

  • KCS - All supported

Keywords: Fax, Link, CSI, Calling station identifier, recipient verification