The Opus codec ( http://opus-codec.org/comparison/ ) can enable very low latency audio. 26.5 ms algorithmic delay (the time it takes to go through the algorithm) by default, but can be set to a very impressive 5ms. Digital audio can't achieve the near no-latency analog POTS has, but it can get pretty darn close e.g. if Opus is used in the very low latency mode. I believe Mumble and maybe TeamSpeak let you choose the Opus latency. I know some other apps like WhatsApp, Wire, and Signal use Opus as well. I don't know what settings they use though.

Another problem is cell phones add ~150ms latency (pre VoLTE) and people that route calls through Google Voice adds an additional 150ms+ latency on top. If you ever talk to someone on a cell phone on google voice, it sounds like they're calling from the moon.

150ms is about the human limit before turn-taking and talking over each other becomes an annoyance. That's why cell-to-landline conversations (150ms one way) were tolerable, but cell-to-cell (300ms one way) conversations are abysmal. And that's all in the same area code. Once you add long distance, it gets worse.