Security as a dimension of trustworthiness in IP-Telephony systems and protocols is a main condition for the commercial success of IP-Telephony. In this work, we present a survey of security requirements and show how various standardization efforts address these requirements. We describe the basic tasks and elements of IP-Telephony systems and compare them to Telephony via PSTNs to derive some possible attacks for example. We classify the security preconditions to achieve trustworthiness of users and providers in this systems. We list weighty criteria for further evaluation of security mechanisms which can fulfil these requirements. After this, we describe the integration of security mechanisms in current IP-Telephony protocols and figure out work areas which have to be solved in future.