SIP has been used to resolve a variety of real problems. The exploitation of SIP by different architectures is what makes SIP successful. A very well designed protocol with no everyday applications would not be very useful. We will see that SIP has been tapped by such architectures as the third gen- eration (3G) of mobile systems and PacketCable-the architectures that will bring SIP terminals to hundreds of millions of users. Third-generation Mobile Systems The third generation of mobile systems has aroused a lot of hype and debate in the telecommunications world. It is seen as the technology that will merge the Internet world and the cellular world. It is claimed that 3G will provide ubiquitous access to all successful services provided by the Building Applications with the SIP Toolkit Internet. Second-generation (2G) mobile phones delivering only voice and short message services between users will be replaced by advanced multi- media terminals equipped with Internet access. Combining Internet tech- nologies with widely available cellular access will make the Internet mobile. The Third Generation Partnership Project (3GPP) develops technical specifications for 3G mobile systems based on evolving Global System for Mobile communications (GSM) networks. The organizational partners of 3GPP are the Association of Radio Industries and Businesses (ARIB), the China Wireless Telecommunications Standard (CWTS), the European Telecommunication Standards Institute (ETSI), Committee T1, the Telecommunications Technology Association (TTA), and the Telecommuni- cations Technology Committee (TTC). The specifications elaborated by 3GPP will be used in 3G systems such as Universal Mobile Telecommuni- cations System (UMTS). Whereas 3GPP develops specifications for 3G systems based on GSM networks, 3GPP2 develops technical specifications for 3G systems based on ANSI/TIA/EIA-41 networks. Both projects, 3GPP and 3GPP2, are part of the International Telecommunication Union (ITU) through the Interna- tional Mobile Telecommunications-200