I've worked on allot of pots lines and haven't done that in 20 years as its all fiber back-hauld now.

The only time I'd send power down a copper line is to fuse it back together so I didn't have to dig it up and replace it and that's super rare now as I'd just put in a order to have it replaced with a 12 pair fiber run.

It has to be a voip connection as there are no true land line connections in the USA. It's probably copper to the house but once you get to the first relay box it runs back via a optical switch and fiber.

AT&T was the last holdover in the far rural northwestern states but they upgraded about 10 years ago. There just not going to say that as fiber is probably whiten 1500 feet of 80% of there subscribers none of the big boys will tell them that as DSL(copper) is easier to maintain and lower bandwidth requirements makes more money in federal subsidies.

More than likely it's just bad chinesium Walmart phones and a very old DSL modem.

Let's try to generate some data to back up your experience.

So, let's test the quality of your internet connection with a long test. Run the following. https://speed.cloudflare.com/ it'll graph the speed and quality of the connection of the internet and wait till it settles. ( Wi-Fi is going to inconsistent compared to a wired Ethernet connection.)

Once the connection looks reasonably stable, then try calling and seeing if the graph changes significantly! Obviously, you can rerun it multiple times to experiment.