Can confirm, when I was doing that forever and a half ago, Comcast actually never gave us trouble when we called them, but anything having to do with at&t or Verizon was like pulling teeth

The worst was one site who was serviced by neither of them, but our last mile provider interlinked through one of them, who linked through the other (I don't remember the order of interlinks) to get to our actual ISP and into our datacenter/WAN. We had issues upon issues upon issues with that site's connection, and it was the interlink between those two that was the culprit. It was easy to narrow down, and easy to fix, but getting them to actually fix it took months of us screaming at our last mile to scream at them louder since we weren't their customers and they both refused to talk to us because of that.

It did get fixed but it took probably a year and a half of fighting before they finally updated the interlink.