At present SIP signalling overload control in next generation networks remains the subject of active research. Usually mechanisms that are proposed for the solution of the problem demand prior knowledge or estimation of their control parameters. The latter can hardly be done without understanding nature of traffic flow feeding system (SIP proxy server) under load. Being motivated by the application of hysteretic load control in SIP proxy servers we briefly present in this paper some results of the analysis of SIP traffic traces collected from telecommunication network. These results can be particularly useful for the estimation of hysteretic loops' parameters.