Statistical distributions are used to model behaviour of real VoIP traffic. We investigate call holding and inter-arrival times as well as speech patterns. The consequences of using an inappropriate model for network dimensioning are briefly discussed. Visual examination is used to compare well known distributions with empirical data. Our results support the general opinion that the Exponential distribution is not appropriate for modelling call holding time. We find that the distribution of talkspurt periods is well modelled by the Lognormal distribution and the silence periods by the generalized Pareto distribution. It is also observed that the call inter-arrival times tend to follow a heavy tailed distribution.