In the paper, we present a procedure to evaluate the parameters of the LEAP (Linear Bounded A-rival Processes) traffic characterisation when a stochastic model of considered traffic flow is available. The procedure, based on an equivalent queuing system, has been applied to a VoIP (Voice over IF) scenario in a DiffServ, (Differentiated Services) network architecture. In particular, we consider the characterisation of a traffic flow composed by the aggregation of telephone sources with VAD (Voice Activity Detection), where each one has been modelled by a fluidic On-Off process with exponentially distributed sojourn times in each state. For testing the goodness of our approach we have carried out discrete events simulations, which have highlighted the accuracy of the proposed procedure in the considered scenario.