Overcharged on MTN contract-more than what I agreed to pay

Good Day

i took an additonal MTN contract on the 06 May 2021. The deal was for a samsung galaxy S21 5G+ Galaxy A32 for R689*36 on MTN mega talk XS( 2gigs +25 MTN minutes). I am being over charged each month, i called in to check last months bill and the consultant said the first month is on a pro rata basis however i see that my bill is over a R1000 this month as well. Additionally i have called MTN numerous times to get help however i keep getting told that they cannot find my MTN number due to a system change. when i call the customer care for general information about my number they are of no help and say that they do not have access to the new system. i am at my wits end. this service is terrible. if thats not bad i see that i have been given someone elses number because i am also receiving their sms's. ***