The unprecedented cellular traffic growth caused by the ubiquitous mobile Internet access has motivated the search for solutions to cope with this explosive demand of connectivity. Mobile traffic offloading through Wi-Fi networks has emerged as an effective way of supplementing cellular services. This approach has been proven very effective to reduce the load of delay-tolerant traffic in cellular networks. Recent developments suggest that real-time traffic like VoIP could be also offloaded if Wi-Fi networks supported a reasonable number of concurrent calls. In this way, ubiquitous IEEE 802.11 networks operating in the 2.4 GHz ISM band could complement the cellular systems in indoor and rural environments. Wi-Fi networks are allocated 14 channels in this frequency band, thereof only 3 non-overlapped channels are allowed for Wi-Fi extended service sets in order to avoid interference. In this paper we investigate the use of assignment configurations with 4 partially-overlapped channels for VoIP transmissions, which may increase the network capacity in terms of concurrent users supported by Wi-Fi networks. Through computer simulations we demonstrate that 4-channel configurations are feasible at the expense of a tolerable degradation in quality of service for voice traffic with pedestrian mobility.