Usage limit not set/ overchaging

I Mrs N Ndabeni, I purchased a Sony Xpeira M4 Aqua on contract on the 22/09/2015. Everything was fine and then on 03/10/2015 the phone was not charging then I took it back to MTN Zevenwatch mall for repairs. I waited almost a month in order to get my phone back, after that I received an invoice for R938.03 although I took a package of R249.00 per month which is affordable to me. I went to the MTN Zevenwatch mall to enquire about the invoice and the lady's name Alicia told me that by the time I bought the phone the usage limit was set by her, but there is a problem on their side she will request a credit memo. My phone had been switched off numerous times because of this. This interrupts my communication to my family and friends. On 16/11/2015 I received a message saying that I must pay R938.03 immediately to avoid full service suspension and legal action. Kindly advise what can I do now because I don't have such amount of money that is why I took the R249 package, and it is not my fault as I was using the phone without knowing that I have reached the usage limit as it was not set in my phone.