Unwilling to accept co-responsibility for huge bil

A week before my statement/bill was received I received a sms stating that I had exceeded the R4000 credit limit of my contract phone bill. I was quite naturally shocked and contacted the call centre. They maintained it was due to high data (I was ouy of bundle). But what they fail to understand is that I on many occasions got a pop-up on screen, telling me to purchase a bundle, which I did (following the prompts) so I assumed that I was safely in a bundle!? The pop-up menu said the transaction was complete so why would I believe otherwise? Now they say it's impossible, as I am a contract user, using a pay as you go pop-up is impossible and it never went through. For pete's sake!? I have since then been contacting them regularly, having to hang-on forever to get to speak to an operator, often getting told they haven't reviewed my bill yet. I refused to pay until they did...more than a month passed. They concluded I was liable as I used a lot of out of bundle data. Now my line is getting suspended unless I immediately pay the full amount, despite my having asked for a payment arrangement. They are the worst customer department in the history of customer departments!!