Intrusion detection systems (IDS) provide security for network systems. They are used in computer networks to detect violations against security policies or unusual events that could lead towards a security thread. Telephone networks based on the internet protocol (IP) called IP telephony (IPT) are a recent development in network usage and will become a common application in the next years as they can provide integrated services based on telephony communications. We identify IPT security demands as well as risks and analyze the possibility of adding IDS concepts to IPT systems. Digital audio watermarking is a technology able to provide different security aspects in this context. The combination of classic IDS and audio watermarking leads to new and promising way of user and data authentication and monitoring of calls.