Mobile ad hoc networking has been considered as one of the most important technologies to support future ubiquitous and pervasive computing scenarios and Internet connected MANETs will be an integral part of future wireless networks. For providing multimedia services and voice over IP in such environment, support for session initiation protocol (SIP) is essential. A MANET is a decentralized collection of autonomous nodes but a SIP infrastructure requires centralized proxies and registrar servers. In this paper, we study the implications of using standard SIP architecture in Internet connected MANETs. We analyze performance limitations of SIP service scalability when centralized proxies/registrars located in the access network are used by MANET nodes. We also present and compare an alternative approach to provide SIP services in Internet connected MANETs in order to minimize the impact of such performance limitations