Users at a branch site report choppy, robotic audio only on calls that traverse the WAN during business hours; internal branch-to-branch calls are clear. The WAN router's egress interface shows increasing output drops in the default queue during peak periods, and a packet capture shows RTP packets arriving with high jitter and occasional gaps. DSCP markings on the RTP packets are correctly set to EF at the phones and are preserved end to end. What is the most likely root cause and appropriate fix?