WiMAX has been at the center of attention in wireless communications during the last years. Nonetheless, very few testbed or field trial measurement accounts have been reported in the peer-reviewed literature. We fill this gap by exploring scenarios where fixed WiMAX is employed for VoIP traffic. VoIP packets typically exhibit large header overheads and small total packet sizes. The actual codec payload per packet is very small compared to the total length of headers appended to each voice frame. Robust header compression (ROHC) can significantly decrease header size by capitalizing on static or rarely changing header fields. Aggregating multiple voice frames into one packet is another attractive and effective way to increase application goodput and overall bandwidth utilization. We study the effect of ROHC and application layer aggregation on VoIP performance in a fixed WiMAX testbed consisting of one base station and two subscriber stations. We find that ROHC increases the number of simultaneous bidirectional emulated VoIP flows by 6% when compared to plain VoIP. When aggregation and ROHC are employed in unison, they allow for 86% more flows than standard VoIP to be sustained in our testbed.