Poor customer service

I cancelled my router contract with Cell C end of March via their Cell C customer services over the phone. Beginning of May I received an statement of Cell C that I still have to pay end of May. I phoned customer service (cancellation dept) on 7 May early morning. They confirmed that my contract is not cancelled. I told them that I already cancelled my contract and it must have been recorded. After holding on for 10 minutes they confirmed there was a call from me on 26 March but it will take time to get the recording. The lady promised me they will phone me back in 48 hours and give me clarity on this case. I trusted them that it will be sorted out but here I am still waiting for the call. Now my question is- do I phone again for the 3rd time at my costs what was suppose to be corrected and cancelled after the first time from Cell C’s side. Time is ticking and Cell C is probably just going to leave it and take money again from me at the end of the month. Really disappointed.