This was the in the UK over a decade ago, back in the bad old days of migration codes and everything being underpinned by BT Openreach who basically had their arm twisted to allow other ISPs to exist and use that infrastructure. And even then they guarded everything jealously (and sometimes incompetently, but that's another story).

We didn't sell telephone lines, so every DSL we sold was on an existing POTS line that each customer already had. Most of our customers wouldn't have known about getting a separate POTSless line even if that was possible at the time. And the customer in question certainly didn't.

They had it installed on their company's one and only telecommunications line. Of all the hundreds of customers I dealt with over the years, it only ever happened the once... although we did have to warn at least one other customer off the idea when it happened to come up in conversation.