It has been starkly noticeable since my company has moved from doing phone-in conference calls to using Teams just how compressed normal phone line conversations are. Combined with a couple of our members having piss poor mobile coverage where they were (but much faster internet) our meetings became a lot higher quality.

On the phone-line side I wonder if that is a degradation due to digital switchovers & compression, a lack of allocated resources, or just pure rose tinted glasses.

VoLTE [0] really improves this, but with compatibility and network limitations make it all too rare. It's like night and day when it works though.

[0] https://en.wikipedia.org/wiki/Voice_over_LTE

I probably make calls with apps more than POTS calls these days. The sample rate and codec on a typical social app is noticably better than POTS calls.

POTS is not POTS any more. Anything other than possible last mile is now digital, with lowest possible if not lower bitrate and old codecs. And that extends to mobile.

Right. So substitute "AMR-NB" for "POTS".

It really should be AMR-WB or even AMR-WB+ by now, and EVS if you are on VoLTE.

EVS are very good at 16-32Kbps bitrate, possibly better than even Opus. But generally speaking Social Apps uses higher bitrate you get better quality. Although latency tends to suffer a bit.

Wait, you're using a smartphone to phone ? how peculiar.

The highest bitrate EVS codecs finally get there I think. Though you'll need sufficient signal strength and networks which support them at both ends of the call.

its likely due to the phone companies squeezing the last bit of space out of their frequency bands, making it sound worse and worse.

Might be a VoIP thing?

At least in Germany, since we got VoIP the voice quality is absolutely crystal-clear if all devices on the line are digital. It will still sound muffled if you connect e.g. an analog phone to your VoIP router, but a DECT phone over VoIP will sound so bright and clear that it's almost frightening.