Phone upgrade and I'm very disappointed with Vodacom

I'm really upset with Vodacom. I upgraded my contract last month and the consultant made a mistake which is now going to cost me R4200 instead of R399pm that l signed for! I have been calling with this query to Vodacom but it seems no one is sorting out this problem. After paying off this contract, l will never again upgrade my phone with Vodacom again. l have been so loyal with you guys never missed installments and this is a third upgrade but in return thats the service l received