Just to put this in perspective, a traditional phone line encodes 56 Kb/s of data, which was believed to be the size channel to send the human voice with a reasonable quality. They are doing it in 1.6 Kb/s!
G.711 (which is standard now for non-cellular call audio) is a step down, but Opus at 16Kbps sounds better to me than a classic, full analog system due to the lack of band cutoff & smarter encoding.