I have 1 VoIP phone and a EdgeRouter Lite 3. Rather than do explicit QoS, I setup the smart queue with hybrid and fq_codel. It made the buffer bloat score on speedtest.dalreports.com into an A+. Given how fq_codel works and how little bandwidth VoIP uses, I think it is rather unlikely that the VoIP traffic would benefit from stricter QoS.

On the other hand, if the VoIP phone were somehow compromised and began flooding the WAN interface, it would not starve other traffic with the smart queue. I suspect that unconditionally making the traffic a high priority might cause a compromised VoIP device to starve others for traffic.

There are trade offs to both approaches. Combining them in a way that the VoIP traffic is always prioritized up to a certain amount of throughout and forced to contend with other traffic beyond that would be ideal. It might be possible, but figuring out how to do it when I had the good enough solution of the fq_codel lead me to just use that. If someone figures out how to prioritize only up to a certain amount of throughput (e.g. 20kbps), I would be interested in how it is done.