Service from Hell!!!

Infuriated by MTN and Mondo. They offer a laptop contract and when it suits them, they want out and now I must bare the cost. I accepted the contract from their sales per at a premium less than R200. a month later, after using the laptop, I threw away the box and then boom, the bill comes and it's double the agreed amount. I query it and receive feedback after investigation that my compliant was valid. I'm now offered to return it and they cancel the deal. Now I don't have the box anymore and because of that, they won't take the laptop back but continue charging me on the amount that was not agreed to. I stopped paying and they suspended my cellphone number (another contract) because this is now attached to the new laptop contract. I asked to cancel my cellphone contract and paid amount that their retentions team asked me to pay and sent pop. 5 days later, there's no response, my number is still suspended and when I call mtn, now the amount I paid is not reflecting on their system and I owe more than it was stipulated on email by their retentions team. I really don't understand, why they can freely change terms when it suits them but not willing to meet me halfway because when I threw away the box, I had no idea that they've made a mistake on their offer. This situation is serious pain and I just wanted out but their service is frustratingly slow and poor!!!