In this paper, a hybrid network consisting of both WiMAX and WiFi links is set up and used as a testbed for Voice-over-IP (VoIP) performance studies. Relevant metrics are defined and used to analyse the performance of this hybrid network and its ability to support VoIP calls. End-to-end delay and packets loss are measured as a function of number of ongoing VoIP calls. Based on these measured results, we propose a procedure to evaluate the maximum number of ongoing calls can be supported by the hybrid network while maintain the call quality at the desired level using the ITU-T specified E-model. The experimental studies show that this hybrid network is capable of supporting real-time VoIP calls, and that the network is able to support up to 12 simultaneous G.711-based VoIP calls and more than 20 simultaneous G.729-based VoIP calls. While it remains a challenge to ascertain the exact reasons behind the low network utilization, the proposed procedure for determining the maximum number of VoIP calls which meet the QoS requirements will be useful and valuable to service providers with the intention to provide VoIP services over such hybrid networks that interoperate these two technologies in the near future.