A complaint regarding my contract phone

I had recently took a contract phone Oppo A58 at the MTN store at evaton mall however I am not happy at all with the billing invoices. Two days after I took the phone this was on the 17th of March 2024 I received an invoice which was not on the signed contract. I went to the store to have the issue resolved but they said I must pay the bill first which was surprisingly high and when I went to the store I was shocked to see that my contract was for 24 months on my client profile however I was issued a 36 months contract. I was then assisted and they said they will reduce the amount to R359 for 24 months however I was not given a new contract stating this and on the day of the debit order the debi check was no longer there now I was not able to pay the first bill I then waited for the second bill it also came with an invoice with a different amount to what we agreed to it stated that I received data on the days when I did not. I contacted their complaint site but never received any feedback from them, I then called but the consultant hung up the call as soon as I mentioned reporting the matter as *****. I am very concerned as I cannot pay for something I do not understand and it will affect my credit score because even now I do not have any debi check or access to pay the bill. They say I owe R1100 in a period of a month for a phone that I'm supposed to pay R359 for. Can you please kindly assist me in getting my contract and the correct billing.