Vodacom sneaks in an extra debit after contract cancellation - again

Having had no joy in resolving the belated cancellation of my step son's cell contract when we was transferred to Germany I decided to cancel my contract with Vodacom. On 12th November 2017 I requested a premature cancellation and received a quotation for R4,177,23 to be paid within 14 days. I paid this on the 16th November and emailed them the signed quotation, a copy of my ID and proof of payment. As expected, nothing more was deducted from my account at the end of November. Then on 29th December a debit of R738.99 came off my account. I contacted my bank and had this reversed. Since then I have called (nightmare getting through), emailed and have never ever had a response. I then received a call from an attorney and I asked her to email me her details and I then forwarded her all my correspondence. On applying for my new credit card I am now advised that I have a negative credit rating from Vodacom therefore my credit limit has been reduced. Is this how a spineless company resolves their issues before replying to numerous emails and phone calls? They get so much negative publicity on talk radio I cannot believe that they have not yet been fined as MTN was in Nigeria. I certainly don't hold any of their shares as an investment.