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The next generation of wireless networks (4G) will use OFDMA (orthogonal frequency-division multiple access) technology on both forward and reverse links. In addition to high speed data services, these networks will also support voice using the voice over IP (VoIP) protocol. In this paper we focus on the capacity of VoIP service for the OFDMA uplink. Although resources allocated to users within a sector are orthogonal, transmissions are affected by interference from transmissions in neighboring sectors. Therefore the user capacity is determined by the uplink bandwidth, the maximum transmission power of the mobile station, the uplink channel conditions, the background noise and the intersector interference. As the transmission power of each mobile increases, the interference caused also increases. Eventually the cell edge users can no longer support the rate required for the VoIP connection and goes into outage. For a given resource utilization per active user we determine the maximum number of users that can be supported. We first determine the centralized solution and then show that a simple distributed algorithm can also achieve this optimal solution. We then show how user capacity can be improved by varying the resource utilization of users based on their channel conditions. Numerical results are provided to illustrate algorithm performance.