You didn't by any chance have an account at one of the previous ISPs that @lantic bought a few years back. I can't remember what they were called, but this has happened a few times now. People cancelled at those ISPs, @lantic's takeover bollocksed the records, and a few people have been told they owe money for a service they haven't used or had for years already.
Also, to anyone in the market for an ISP, stay away from @lantic. Klomp skelm bliksems.