Left Vodacom with zero owing but now they demanding money

My contract ended November with Vodacom, I requested that my number be moved to prepaid, once that was done I ported my number away from Vodacom.

I received an invoice where Vodacom owes me R6.99 but I am receiving calls from MBD attorneys requesting money from me for my Vodacom account. When I told them I don’t have an account they just say I must call Vodacom or they say ok but they continue to phone for more than a month.

Why is this happening because I left Vodacom and no longer wanted to be a client. Only invoice i received since moving was the one that Vodacom owes me