Forced to pay for 2 data SIM

I received a call from a lady working for MTN, she sold me a data SIM contact worth R99. I received two Sims, the delivery guy said he didn't know why, I opened 1 and used it, then I received debit order mandate for R400 then I wrote an email on the 18/05/2022 disputing the mandate but no one called me back, I was then billed R343 on 30/05/2022, I again wrote another email in June but still no one responded then I called customer care then I was given a number for the company which sold the SIM on their behalf but their number doesn't go through. Now the bill is R756 & I refuse to pay that amount because I feel I am being ******. MTN customer care is useless cause they fail to even reply an email. Now my credit score is being affected because they are overcharging me. Regards Lungelwa Ndlovu