I’ve never seen it in the US. It’s always been two options, buy from the company that owns the phone line (DSL)/fiber to your house, or buy from the company that owns the coaxial line to your house (cable).

That wasn’t always true at least for POTS, I think it was called “dry DSL”? The phone company owned the line but your internet was through another company.

they used to, that was part of the debate about title-2 or not and net neutrality. That's why when DSL was the fastest internet there were so many choices and options but now it feels like there are hardly any (depending on where you live).