No service from Standard Bank

I contacted Standard bank during April to voluntarily increase my vehicle repayment. They agreed but deducted the incorrect amount in April. I assumed the error would be corrected in May but no, they debited the repayment only and not what they deemed as arrears. Imagine my surprise when I was contacted by a collection lawyer last week who very aggressively wanted me to make arrangements to cover the arrears. As if somehow it's my fault that Standard bank deducted the incorrect amount from my bank account. No contact from Standard Bank in this entire process whatsoever. Not an SMS, phonecall or email. NOTHING. Get your house in order and make sure to collect monies as agreed. Also please ensure that this is not indicated as a missed payment with the credit bureau.