Vodacom want more money from me after I paid them

I have been with vodacom for years and I have cancel my contract with them and each and every time I'm been prove it was the the right choice. My invoice shown that I'm paid up. But when I have done a Sim swap I'm been told that I have a outstanding balance that I from June but each month my invoice amount is being paid. Now I'm going to loose my airtime because out of the blue Vodacom airtime last only 60 days.