VODACOM billing is a shambles

I upgraded my phone 6 weeks ago and was told that my first installment for the new upgrade would be at the end of August and would be for R429 in total. When accessing the Vodacom app, I see that the total is now R738.71 due at the end of August which is impossible as I still have free minutes and data remaining and have not made any other purchases. This evening, I received a statement from Vodacom to say that I now owe R1,032.99. I find this absolutely insane and as a Vodacom customer for over 10 years, incredibly infuriating. This is a waste of time having to keep following up with Vodacom because their correspondence is not up to date. Kindly rectify this and send me a bill with the correct amount of R429. DO NOT DEDUCT incorrect amounts off my account.