Neotel- Worst service

I cancelled my contract in September 2018, so one year ago already. They sent me an invoice which I queried and waited almost 6 months to get any answer from them. I want to settle the outstanding amount and have been asking for a final invoice and banking details for a month now, to no avail. I find that they are one of the companies that has the worst service, that is why I cancelled my contract with them in the first place.