Major standards organizations, ITU, ETSI, and 3GPP have all adopted SIP as a basic signalling protocol for NGN. The current SIP overload control mechanism is unable to prevent congestion collapse and may spread the overload condition throughout the network. In this paper, we investigate one of the implementations of loss based overload control scheme developed by IETF work group which uses hysteretic load control technique on the server side for preventing its overloading. Two different approaches to calculate performance measure of SIP server are introduced. We follow an analytical modelling approach to construct and analyse SIP server model in the form of queuing system with finite buffer occupancy and two-level hysteretic overload control. The formulas for stationary probabilities and the mean return time in the normal state of space were obtained. Simulation is the other approach which allows to eliminate disadvantages of analytical modelling. At present, there are no simulator for modelling the work of SIP servers in overload conditions with an application of mechanisms which are currently under development by IETF. Approaches to its programming implementations which reflects the protocols and functions which are fully or partially built into the original SIP systems are proposed in the paper.