In emergencies, telephone calls provide a sense of security because one hears the other person's voice directly. Calls play this role when they meet the QoS requirements. During emergencies, numerous VoIP sessions arrive in the network, thus session admission control must be performed while meeting the QoS requirements. Session admission control requires a multiple sessions traffic model, which is obtained by the superposition of single session traffic models. Voice payload size varies for single session traffic. We propose a single VoIP session traffic model for variable voice payload size. We also discussed the appropriate voice payload size selection in terms of the average bit rate.