Being billed again for cancellation!

I paid my cancellation fee to Cell C to terminate my contract in FEBRUARY as I've relocated. Cell C happily accepted my cancellation fee but continue to debit my account. I have just received another cancellation billing fee for the month of May (***)<br> <br> I wish to be credited with the amounts taken for March/April.<br> <br> Outstanding customer service number (nobody has responded to me) ***