T 5.12 Interception of telephone calls and data transmissions

By abusing user facilities, it may be possible for colleagues to listen in on telephone calls. One example is the add-on (three-party) conference. If subscriber A receives a call for subscriber B, he might try, in secret, to establish a three-party conference, instead of passing the call on. Subscriber B would not be aware of this fact if he had a telephone set without a display.

In addition, it is possible for third parties to listen in on calls by activating disabled user facilities which are partly not allowed in Germany. One example is the add-on witness feature. Such an activation requires in-depth knowledge of the system.