Error by contracts department that's cost me R400

I have a data contract by Cell C for 2g p/m. My wife has started working from & I need more data, so I contacted my current service provider. A lady told that I could upgrade my current contract to a 5g p/m deal. She told me that because there is no handset involved, that she could cancel my current contract & activate my new 1 immediately all I had to do was pay a pro-rata. I agreed to it & thought that was that. Then today I decided to check what my current usage is. I was horrified to discover that not only was that upgrade not activated, but I had run up a bill of R250 & now I need to buy more data to get me to the end of the month. This error is going to cost me +- R400. I have cancelled my upgrade & will no longer be doing business with Cell C. This matter was handled terribly. I don't know the name of the person I spoke to or the exact date, because to be fair I have always received great service from Cell C & I didn't think it would be necessary. I'm extremely disapointed to say the least & will now be ending my 10 year relationship with them.