Cell phone audio traditionally only covered 300Hz-3.4kHz before being lossily compressed down to 4kbps (or sometimes higher, depending on network, load in that service area, etc). That is complete shit. Recently, there have been other protocols adopted with greater bandwidth (all the way up to 7kHz, which is still several khz short of covering all the content of speech, but considerably less terrible) and less compression, but if you have audio that's actually good and not merely passable, it's probably because your phone is actually transmitting audio as voip, with a much better codec then is used for cellphone audio transmitted over the standard channel.

G.711, the standard encoding for home phone systems since they went digital, is usually filtered at 300–3400Hz as well. Chances are if you had a home phone in the 80s or 90s it was filtered at 300-3400Hz somewhere along the path.

And the history of that filtering even predates digital lines, due to frequency multiplexing: https://en.wikipedia.org/wiki/L-carrier

Ironically, you've got a better chance of getting acceptable quality on mobile-to-mobile calls these days than when calling from a landline:

The big mobile carriers actually have VoIP interconnects preserving wideband audio, while connecting to an (especially smaller) landline carrier might still involve a circuit switched path (going to the physical location of the area code dialed, too!) that inevitably forces everything through a 4 kHz, 8 bit bottleneck.

It’s different audio codecs and data connections which can be changed and adjusted.

Voice over mobile data will benefit from a different type of data compression relative to how data packets are handled in a cellular radio vs a wired internet.

The landline service could be as clear or clearer than mobile data, it just isn’t configured to do so in your case. I have seen VoIP setups using a high quality codec and a handset that can use those frequencies.

Phone companies use different setups that compress to their advantage for many more callers.

I'd not really call it different packet handling (except for some of the earlier, mostly 2G ones not providing FEC at lower layers and delegating some level of error concealing to the codec, as far as I remember):

The main difference is that the bandwidth available was just much lower, so mobile codecs are compressed more. (Satellite phones take this to the extreme – 2.4 kbps is a typical data rate after compression there!)

But so were e.g. international trunk lines; they squeezed a lot more than one voice channel into 64 kbps using compression, silence suppression etc.

> The landline service could be as clear or clearer than mobile data, it just isn’t configured to do so in your case.

An analog landline has relatively little chance of ever gaining wideband support, since that would require swapping out line cards at the provider, and the trend seems to be to get rid of these entirely (in favor of a VoIP adapter in the CPE).

I think I've once used "HD voice" on a "landline" when calling a mobile phone, but that only worked because my home router was actually doing SIP in the background.