Improving call quality
In short, the best thing you can do to improve your call quality is to turn off your Wi-Fi when making a call and use your cellular data connection instead.
The way that Wi-Fi works makes it especially susceptible to packet loss and jitter (basically choppy or and uneven audio connections) when making internet-based phone calls like those in Unlisted.
You may have what appears to be a great internet connection, but because transmitting an audio conversation is more complicated than loading a webpage you may not tolerate the same level of service.
In very general terms, in any given call each of your voices are chopped up and sent back and forth and they need to arrive in the same order exactly and with good timing.
If a webpage loads an image on the bottom before an image on the top, you don't care as much. But if the end of your word arrives before the beginning...that's weird.
For more technical reasons, this is much much likely to happen when you are on Wi-Fi than on any other network. So if you run into bad calls, try turning off Wi-Fi for your call and it may work much better for you!
Curious about your connection?
Here is a free tool that you can use to check your connection: https://networktest.twilio.com/