Telephone networks form the oldest large scale network that has grown to touch over 7 billion people. Telephony is now merging many complex technologies (PSTN, cellular and IP networks) and enabling numerous services that can be easily monetized. However, security challenges for telephony are often neither well understood, nor well addressed. As a result, telephone networks attract a lot of fraud. In this talk, we will systematically explore the fraud in telephone networks, focusing on voice telephony. We will present a taxonomy of fraud, and analyze two prevalent fraud schemes in more detail: looking into the ecosystem of International Revenue Share Fraud (IRSF), and discussing a new countermeasure to the well-known problem of voice spam.