Bad service Vodacom

We have been happy loyal Vodacom customers for over 20years, until recently. We upgraded 2 contracts late last year, with 2 payments to go on old devices. Go to the 3rd month and still charged double. Query done and found that the fault is on their side. End of January our bill suddenly R600 more than usual raking into consideration we have top up contracts so no reason for the bill to be more, no payment skipped. We have a contact due for upgrade and decided not to do device upgrade now so in fact invoice must be R300 less. Needless to say, they do not find the problem operator not assisting or even listening and they locked the line demanding immediate payment of the R600 with no explanation why the invoice is more with the ammount and even now still more, even considering the plan increase our monthly invoice should be less because of the device that we did not upgrade. Our branch went through everything with us and see it the same s us but headoffice does not find anything out of place