But 56k dialup (actual speed more like 25k) is too slow to load an https certificate before most sites time out. You aren't going to be able to load google.com

No, it is not feasible to run modem signals over VOIP, as the various codecs all compress signals and cut frequencies and all manner of things to reduce bandwidth consumption, which are incompatible with modem signaling. You could get away with it in a homelab for fun, but you have absolutely no control over what VOIP codecs e.g. Comcast is running, so it is effectively impossible. Even if the phone company says they can offer you a copper line, your copper line will eventually get converted to VOIP at the end of the street or wherever, and then it's up to whatever commercial provider you're paying to choose the codecs for VOIP, which are never modem-friendly. I worked on this stuff about ten years ago. There are fax codecs but they are very hard to get working reliably.

AT&T used to be the default landline provider for my address, but they recently got the regulators to release them from that responsibility, so now there isn't one. So I can't buy a landline for my property, even though there's copper running right past it and a pedestal where all they'd have to do is reconnect the line to my house. If I call AT&T, they'll sell me a cell phone, but not a landline.

Fortunately I have fiber (from another company), so it's not a problem; but the concept of being able to get a landline anywhere is going away.

Where I live in Colorado there is literally no cell coverage by any cellular provider. No 5/4/or 3G coverage in miles in any direction while outside and no matter how far up the mountain behind my house I climb.

Their maps claim there is coverage, but there is not, and they don't really care that its not true.

Rural mountainous areas have very bad cell coverage. When I grew up the local Verizon store didn't actually get signal and you had to drive up the road from there to take calls.

Those are the kinda places I imagine are expensive to run new installs to, so it's really phone lines or satellite

Here is "ping 8.8.8.8" showing latency over cellular internet some of the time, and I live in the centre of a city:

  64 bytes from 8.8.8.8: icmp_seq=6145 ttl=114 time=363613.635 ms
  64 bytes from 8.8.8.8: icmp_seq=6175 ttl=114 time=334289.726 ms
  64 bytes from 8.8.8.8: icmp_seq=6176 ttl=114 time=333689.274 ms
  64 bytes from 8.8.8.8: icmp_seq=6177 ttl=114 time=332851.621 ms
  64 bytes from 8.8.8.8: icmp_seq=6178 ttl=114 time=332673.845 ms
  64 bytes from 8.8.8.8: icmp_seq=6179 ttl=114 time=332618.215 ms
  64 bytes from 8.8.8.8: icmp_seq=6180 ttl=114 time=331634.496 ms
  64 bytes from 8.8.8.8: icmp_seq=6181 ttl=114 time=330736.758 ms
  64 bytes from 8.8.8.8: icmp_seq=6182 ttl=114 time=331050.087 ms
  64 bytes from 8.8.8.8: icmp_seq=6183 ttl=114 time=330813.820 ms

Many rural areas have no cellular service. I vacation in an area where Satellite, landline phone service, and some very bad DSL service are the only options. Since it's a vacation spot, we opt not to use the internet there, but there are people who live there.

In very rural places, they may only have edge or 3g at best, if they have any connection.

No idea why you're being downvoted. I can right now today call AT&T and get 300Mb cellular internet for my house. It's $65/mo.