When they buy a debt, they rarely get comprehensive documentation.
It's far from a good solution, but it should at least put you in a better position vis-a-vis the courts if it comes to that.
Furthermore, there's an entire category of people for whom "talk on the phone" is not an option, period. If they wanted to cancel by the approved method, they'd not only need to take that time themselves, they'd need someone else willing to take that time with them.
I've seen Americans living in Europe get bitten by this quite frequently in some countries, where sending something to collections is both commonly done and the collection agency will successfully take you to court and win in almost all cases even for pennies.
On the other hand, the EU has had "two-click cancellation" regulations for a while now, so there is a better alternative available for both sides (customer and company).
If you misrepresent that you have and they are just ignoring you, you might practically get away with it, but do know that that kind of misrepresentation might get you into trouble some day.
If however the merchant is actually unreachable for a bona fide cancellation request, that's totally on them.
Disputes should have been the backstop to incentivizing the market to making cancellations easier, but things apparently didn't pan out that way. So arguably, the free market has had its chance, and it's time for regulations.
"I attempted to reach them via phone on or about _____, ___. No remedy was achieved."