Trunking makes it unlikely you'll achieve much more than 56k without the phone company being "cooperative" (setting up DSL). Even 56k is/was a major feat given that the analog signals would typically get digitally compressed into 64kbps channels.

If you could somehow get a "pure" un-trunked copper line end to end, it'd be down to distance and how clean the switch connections are - e.g. back when I was running an ISP in the mid 90's the local telco often delivered multiple Mbps leased line connections by simply hard-wiring "normal" copper subscriber lines on the switches in between, but of course that let them test and ensure the connections were clean enough and take appropriate action if they weren't.

The 56K modems of 20 years ago were near the theoretical limit of bandwidth on a conventional phone line. They required the far end (the ISP) to have a digital connection to the telco; i.e. you can't have 2 56K modems call each other and connect at 56k.

Depending on the telco setup; your analog telephone call is encoded into 64k digital, and some signaling schemes used one of the 8 bits for signaling, leaving only 56K for digitizing the audio.

For analog-to-analog calls the best rate was 33.6K.

https://en.wikipedia.org/wiki/Modem#Using_digital_lines_and_...

you mean *DSL? The issue lies with the copper pair and the frequencies used. And of course the hardware in-between. So between 10 and 120Mbits/s.