Pathetic service, billing issues

I upgraded my cellphone contract on the 30th March and wasn't advised that for the month of April I would have to pay the full device fee for March (even though it was one day before month end) and then the full contract fee for April. I feel they should educate the client and advise that if you upgrade 1 day before month end you will pay for a full months device fee and pro rata on any data/call packages and rather advise the client to wait until the next day and then upgrade to avoid a double billing of A device u had for one day. I was not told this at any point, the agent in store only advised me that I would pay a pro rata amount for March but it won't be a lot because it's only ly 1 Day. Had I known that I need to pay the full device fee I would of waited until the 1st of April a day then upgraded seeing that it was literally a few hours away. Telkom also decided to bill me in full for the outstanding amount for my old device even though it was agreed that I would pay it off in monthly installments. I called into the call centre and spent almost 30 mins on the line with the agent, only to be told there is nothing they can do, I must pay the full device fee and both device fees, bearing in mind this is for 2 contracts. Pathetic service from Telkom.