uce the size of routing tables) and to benefit from its auto configuration features. Furthermore, issues related to IPv6 migration strategies have been investi- gated and discussed exclusively in the IETF. Several working groups have thus been chartered to work on operational and migration mechanisms (e.g., v6ops Working Group). Nevertheless, this effort focused essentially on the migration of connectivity services (i.e., IP transfer capability) and not the services to be deployed over an IP infrastructure. Concretely, problems related to migrating "high level" services such as Voice over IP (VoIP) and TV broadcasting, built above an IP infrastructure composed of heterogonous nodes (i.e., IPv4 and IPv6 ones), didn't benefit from a large amount of investigations by the Internet community. The rare initiatives that have been launched were about Domain Name Service (DNS) and File Transfer Protocol (FTP) concerns. This effort dealt mainly with developing Application Level Gateways (ALGs). From this perspective and as far as VoIP is concerned, service providers should evaluate the impact of IPv6 migration on their service offerings and associated architectures. This chapter tackles the case of telephony over IP service offerings and the impact of the introduction of IPv6 on deployed service platforms and architectures. Considering the panel of telephony over IP is diverse and rich, this chapter focuses only on SIP. Within this chapter, and if no explicit reference to IMS is quoted, a "SIP service" (or also "SIP-based service") denotes a SIP-based telephony service involving at least a SIP proxy server and a Registrar. In such a service, the proxy server and the Registrar have IPv4 or/and IPv6 connectivity. These functional entities could parse IPv4 and/or IPv6 addresses but could not use all of these IP versions to transfer IP data. A service provider offering a "SIP service" provides its customers with relevant information in order to reach SIP entities, especially the addresses of outbound and inbound proxy servers 