Voice over IP is widely seen as one of the next killer applications, integrating data and voice networks as well as applications. However, proprietary voice networks and components also offer good degree of separation and segmentation, increasing confidentiality, integrity and availability of telephony services versus data networks. When these two services are integrated (and in many places they are already), new threats arise and additional risks have to be mitigated. There is no doubt that also the PSTN has seen its share of attacks (war dialing, phreaking, fraud). Migration to VOIP on a large scale will see similar and new types of attacks to the combined infrastructure. It is recommended to separate the two networks logically (or even physically) and conduct an in-depth review of the threats to VOIP and implement effective countermeasures to ensure availability of voice services. The cost savings achieved by using VOIP will mostly be used to implement security.