Sometimes the key to troubleshooting one problem is to find out that there are other problems. I was informed that some phone calls had an echo after the local staff person spoke. It took some effort to confirm that it was an echo and not just a delay (although latency can evolve into an echo), and so I thought that the problem had been sufficiently clarified. However, while looking at echo-related configurations, someone mentioned that they experienced the echo more when they had to turn the volume all the way up.
Of course, louder volume causing more echo did not surprise me, but it did send me down a different path. Rather than troubleshooting many different echo-related settings, I focused on how to help them avoid turning the volume up. I turned Network Jitter Level from High to Medium to slightly reduce network latency, and I increased the PSTN to SPA Gain to 12 (values range from -15 to 12). According to the first link, the echo suppression is an auto-gain that overrides the PSTN to SPA Gain, but that was not our experience. Increasing the gain addressed the volume problem, and the echo problem is addressed for the moment.