Mtn magical charges added to account

So I have had a contract with Mtn for almost 10 years. The contract is in my husbands name. Every year or so my line gets suspended. Then when we query the suspension, a magical amount appears that we apparently haven’t paid. Now I know how these people work. If you shortpay your account they will suspend the line within 2 days. So now I’m the middle of lockdown my line gets suspended. No warning. I try to get through to them on 135 but they are having technical problems. So I send emails. They het back to you after about 3/4 days, ask for verification details which take another couple of days. I have been paying this account myself and I know I have not skipped any payment anywhere. But magically I owe them R900. They can’t tell me where this amount comes from and if I check the statements this amount has apparantly been there since December 2018. I’m on a top up contract so my contract can’t accumulate like this? So now? After a year and 5 months? Please explain mtn. I’m due for an upgrade and I definitely will be merging to vodacom. Thank you for nothing.