Worst Service ever experienced

In December I signed up for a 6 month contract with Mweb & the contract came with a free modem to which I specifically stated that I already had a modem but I was advised that I could use the other modem as a back up. To my surprise they stated billing me for the modem on the 6th month of the contract. I started querying these charges & I was told a manager would call me to which he did I advised him that because they breached the contract and I was not happy with their services anyway they can cancel my contract with immediate effect, he advised that he was going to listen to the recording and come back to me. After a day or two he came back advising that the recording was not clear & that they will collect the modem & cancel my account, they collected the modem still sealed in the same condition I got it & I thought the matter was resolved I received a call after a month advising me that I was still owing +-R400 on the account to which I responded that the matter was resolved & the consultant did some checks & promised that a Manager would call me that was in June now I received a letter of collection without them contacting me I owe them nothing they breached the contract