Pre-cancellation of contract gone wrong as usual!!

Before I immigrated, I paid to cancel my contract ahead of time as my contract had not yet come to an end. I made the payment and my contract was cancelled 2 days later and I was then switched to prepaid before the end of the month. I had to buy additional airtime and data so I could use my phone until I left the country on 17th March. My boyfriend cancelled his and paid the cancellation fee and the months left on his contract too but closer to the end of March. We have BOTH been sent invoices for this month even though we have paid the fees and cancelled the contracts!!! I cannot believe it. What is the point of paying and going through the process if we are now being charged again???? I cannot call the call centre as I am in another country and now I could risk being blacklisted because Vodacom cant get their sh&% together! I even insisted to have the cancellation in writing as I knew this would happen. How can I get the invoice reversed and proof of the error now?!