If telephone calls or data are transmitted in an unencrypted form, there is a general risk of attackers eavesdropping on or reading the information. For example, they could tap directly into the telephone cables or eavesdrop on a PBX system when it connects callers. With VoIP, it is even easier to eavesdrop on telephone calls and data transmissions than with classic PBX systems. All the voice data is transmitted in a media stream—using the Real-time Transport Protocol (RTP), for example. In the case of VoIP, techniques such as spoofing and sniffing enable attackers to use all types of incursions that pertain to data networks, as well.

In many PBX systems, callers can leave a message for recipients who are not available at the time of the call. Some answering machines, especially those in VoIP systems, send this information in the form of an audio file attached to an e-mail. The contents of such e-mails could be directly intercepted and heard by an attacker.