For years, telephony service providers have supported voice calls using circuit networks controlled by ISDN User Part (ISUP) signaling. ISUP signaling is based on a mature standard, is widely deployed, supports a variety of services, and has grown to include extensive congestion control mechanisms. With the move to packet networks, Session Initiation Protocol (SIP) signaling is being introduced to support voice over packet networks. As service providers begin this migration, comparisons between SIP and ISUP signaling are inevitable. Specifically, service providers want assurance that SIP provides congestion control mechanisms for packet networks similar to those ISUP provides for circuit networks. This paper is a result of a study to anticipate the congestion control needs of packet networks based on Signaling System 7 (SS7) network experience. It provides a comparison of SIP and ISUP congestion control functions as well as an analysis of mechanisms that could be deployed in SIP networks to enable them to achieve parity with ISUP networks.