I'm having not dissimilar problems with my former phone provider, Orange.
Despite their promising me I could change my tarrif when I needed to, they refuse, 2 years ago, insisting I see out the term of my contract.
That was due to end in September. I cancelled in August.
In September, they denied they had received any cancellation and insisted I make a new one, starting then.
Now, they have issued me with a rather large bill, claiming I owe them for unpaid line rental, since the date of the latest cancellation.
Apologies if I seem to be hijacking your thread, I make this point in sympathy.