Received a legal letter for a fully paid and cancelled contract

Extremely disappointed... I cancelled my contract prematurely, and paid the full outstanding amount as required by the cancellation procedure. I paid all my dues to cancel this ****** contract... Now not even 3 months later, do I receive a legal letter from Vodacom's attorneys that my account (that I cancelled and paid in full) is in arrears. They made the cancellation process hard and expensive enough, but to still be sued for outstanding fees that are nonexistent is borderline ******** of them. Have tried to call in so many times to settle things, but their automated system won't even recognize my entered contract number when phoning them. What pathetic service from a ******** company... I'm truly sad that I devoted 5 years of my life working for them and earning them millions in overseas revenue... For what? Just so they can ***** over the working man?