Billing and communication @ call centers a problem

I had a Vodacom handset contract that I decided to cancel because of problems I had with them. I went prepaid on my handset after the expiry of the contract to avoid these contract hassles. I asked Vodacom to give me the full cost of discontinuing it, which 'they did'. I paid and they said I was clear to migrate to prepaid the next day. I got a modem instead and around a month down the line I got a bill of around R4000 for data and R1000 for a phone contract I had stopped and paid for. When I consider the bill on the modem which I got it doesn't make sense because 1) I always had a password and disconnected the modem from my laptop every day and 2) I never download/upload data consuming items/programs. I've grown tired of the fighting and will pay the data fee BUT not the handset one. I made a payment arrangement with a certain call center agent called Philisile and got a reference no. (S3-GK220-0HV8L) to resolve things but Vodacom agents have still been calling me at awkward hours and they refuse to call me at 13:00 during lunch time due to the nature of my work. Instead the call center agents say I must call them but I never get through and this can affect my credit record. Help