The rapid development of services provided on SIP networks not only define the necessity of creating new equipment and standards but also requires the development of new methods and programming software tools for modeling and analyzing the effectiveness of the overload control mechanisms in SIP-server networks. The modeling process utilizes mathematical and simulation models as well as simulators. Only simulators or simulation tools make it possible to solve problems related to the analysis and optimization of the control parameters. The most appropriate modeler is the simulators reflecting the protocols and functions, which are fully or partially built into the original system. At present, there are no simulators for modeling the work of SIP servers in overload conditions with an application of mechanisms, which are currently under development by IETF SIP Overload Control workgroup. Simulation tool that supports the main SIP RFCs such as RFC 3261 are proposed in the paper. The architecture of SIP node is developed, and numerical example is presented. © ECMS Valeri M. Mladenov, Petia Georgieva, Grisha Spasov, Galidiya Petrova (Editors).