Shameful behaviour! Contract cancelled, now handed over without even being given the bill!

I was a Vodacom customer for many years. I have recently moved abroad and will be abroad for the next four years. Before I left I started the absolutely arduous process of cancelling my contract. I was on a small flexi top up contract with no phone included that was up for renewal in November 2019, but expiry in February 2020. I called the useless cancellation line and was given a quotation for the early cancellation of my contract. I duly paid this and did as was required, sending proof of payment, signed documents and copies of ID’s to the email address provided. I received email confirmation of the job number and finally, email confirmation that it was complete. I waited a couple of weeks then check my airtime balance to find I was still on contract. What followed was numerous calls to the cancellation line, every time speaking to different agents, who advised me to call different departments, and those departments sending back to the cancellation line. Every time I asked to speak to the manager on duty, they were always occupied with another call. I never received the numerous call backs I was promised and finally, right before I was due to leave, after a final call somebody was able to terminate my contract and move me to prepaid. Now after three months, I have received a letter from a law firm-debt collectors- demanding outstanding money. I have not received any invoice from Vodacom, and to my mind I paid all monies and a penalty amount for early cancellation as I was meant to. This is absolutely disgusting service from Vodacom. I am not even in the country able to fight the matter now.