The Differentiated Services architecture offers a scalable alternative to provide Quality of Service (QoS) to the new multimedia applications in the Internet. This paper aims at evaluating the delay and jitter experienced by voice traffic when handled by the Expedited Forwarding (EF) scheme. The analysis includes the effects of different packet scheduling mechanisms implementing EF and of the voice packet size. We also evaluate how efficiently each type of traffic uses an extra allocated bandwidth and the impact of traffic shaping. The results show that increasing the service rate share allocated to the EF aggregate does not significantly affect the competing best effort (BE) traffic. This holds as long as the BE traffic can use the bandwidth left unused by the EF traffic in idle periods.