How to complain about VM making it near impossible to cancel?

Just tried to cancel took 30+ minutes because they take 1-3 minutes between responses and ask lots of pointless questions in an attempt to get you to stay.

Was really frustrating.

The chat got disconnected just as they were confirming my cancellation so I'm not even sure if it worked and they've not sent me any email confirmation.

Anyway, how can I raise a complaint that will actually get heard?