information. Ghosting: obtaining cheap or free access to the network through tech- nical means of deceiving the network, e.g., manipulating a switch or database. Premium rate service: this includes setting a premium rate number which receives a certain sum of money from the originating provider for each call destined to that number. There have been cases where office cleaners were organized to call these numbers and leave the phone off the hook all night or where the fraudster subscribes to an operator under a false identity and generate these calls. Roaming fraud: in this case, the attacker might obtain fraudulently a subscription from an operator and use it in high value activities (call selling to expensive locations, Data/3G session, etc) in the network of another operator. Currently, this kind of fraud needs a few days to be detected, SO huge amounts of money can be lost by the providers. It is common that these techniques are combined together in order to maxi- mize the profits. While all of these fraud scenarios can be seen as direct threats to VoIP networks, the VoIP technology also adds additional risks. 1. Service plan misuse: in general VoIP services are offered as flat rate ser- vices. Operators calculate these plans based on average usage scenarios. While such services are intended for personal use only, some subscribers offer this service to other people-family and friends-resulting in high usage and high losses to the operator. 2. Credit card fraud: in this case, fraudsters use the toll free service of a VoIP provider to find out the PIN numbers of a stolen credit card. That is, while sitting in Nigeria for example, the fraudster would call a premium service in the U.S. and that would require the credit card and PIN number and keep trying this until the right number was found. 3. Identity theft: while the access control scheme of the VoIP protocols (based of MD5 hash technology) is fairly robust it can be attacked. This would result in revealing a user's password to the attacker who would then misu