No Feedback for Months

We have had a business Internet account with Vodacom for many many years at a fixed cost. We decided to change service providers and gave them 3 months notice - bearing in mind that we have been without an active account manager for the last 3 years - and that is when our problems started. They increased our billing for the month that we gave notice to almost double our standard amount. When i queried this i was told they will get back to me. I then followed up every three days with no answers, and thus received another bill by the next month for the increased amount. This went on since October 2017. We have now been sent a letter stating that we will be handed over for Debt collection as we have not paid - due to them not giving us an explanation on the escalation in the amount. I phoned the call center after the letter was received, and after almost an hour on the phone a very helpful lady (and she was very helpful) said that she has escalated our problem and gave me a case number and everything and someone from Vodacom billing was going to get in touch with me. Now - a week or more later - nothing still..... What are we to do? Nothing is being done from Vodacom side but we get warned with a Debt Collection process. We never once said we wont pay -just that we want to know why the billing has increased. O yes - and we are now being billed for Feb 2018 as well after our notice was given in October 2017 - 3 months mean Jan 2018 would have been our last billed month.....Vodacom - this is such poor service. If you can call it poor....