The IP Multimedia Subsystem (IMS) is a standardized architecture that defines networks providing both mobile and fixed communications using packets (VoIP) and circuit switching (PSTN, the good old regular phone service). Using IMS, service providers can specify, design and deploy networks assuring compatibility with other IMS-based networks.
IMS also defines the fixed/mobile convergence (FMC) paradigm where users can, for example, transition transparently from wireless 802.11 (WiFi) or 802.16 (WiMAX) networks to PSTN networks using GSM or CDMA. The goal: to place a WiFi phone or video call at home through an IMS-enabled phone and then continue a conversation as one drives to work *but* without dropping the call; with an IMS-enabled phone, the phone should take care of switching between networks without letting the calling parties know about it.
Lucent’s website has a very interesting white paper about IMS and its evolution. We highly recommend it for those folks looking to understand the philosophy behind these various technologies. Have fun!