S 5.79 Protection against call number identification during use of mobile phones

Initiation responsibility: Head of IT Section, IT Security Management

Implementation responsibility: Users

In the GSM network the number from which a call is being made can be indicated to the recipient of a call. Whether this is in fact the case depends on the technical equipment and the configuration of the mobile phones and/or at the network provider's end.

It is possible to prevent one's own call number from being transmitted to the person telephoned by using the call number suppression function on the mobile phone (either for the next call or for all other calls). This option is often found in mobile phone menus under menu option names such as "Incognito" or "Anonymous".

Passing on of call number information can also be prevented on an ongoing basis through the network provider.

If mobile phones and SIM cards are swapped around between users, this can provide a certain protection against the association of call numbers with particular persons, as it prevents a permanent association between user and call number or between mobile phone and user from being deduced. However, the link, for example, to an agency or company, will remain.

A particular person's mobile phone number can be ascertained not only through transmission of the call number but also from public telephone directories, if the number is entered there. When concluding a mobile phone contract, the question of whether and in what form an entry should appear in public telephone directories should therefore be carefully considered. The same applies to the publication of call numbers in internal telephone directories and to the disclosure of these in various applications where entry of such data is requested (e.g. forms, prize draws etc.).