In recent years, the Internet has proven its ability to carry real-time data, including voice. Today, a small amount of voice traffic has already been diverted from the Public Switched Telephone Network (PSTN) to the Internet. If it expands, this phenomenon can completely change the rules of the game for telecommunications. This paper presents an overview of the main technical problems to be addressed for the provision of interoperable services between IP Telephony and the PSTN. The pivotal element of the solution resides in an interworking function. This function is typically implemented in a gateway whose requirements and behavior are here analyzed in terms of signaling and control protocols (control plane), user data transfer (user plane) and management features (management plane). The presentation is structured around these three planes. The control plane defines the set of signaling protocols to be used in each networking context and the translation between them. Detailed scenarios illustrate the signal translation in the gateway allowing for the establishment of a hybrid phone call. The user plane is responsible for adapting the user data to the properties of each network channel and determines the Quality of Service of the voice call in terms of delay and speech quality. In the management plane, the issues of network, service, security and policy management are discussed.