ACCOUNT MESSED UP AGAIN

I cannot understand how it is possible for MTN to mess up our account once again. We have had endless problems with MTN since last year and for a moment it seemed they actually sorted it out. But then 3 months ago we started getting invoices of R4000+. I am so sick struggling with MTN. There is 7 contracts on my name and all of them are top up. My wife's account has jumped from around R300 to more than R2000.00 the last three months and according to the invoice it is for packet switch data calls. We(my daughter and the MTN branch) has logged endless queries with MTN but to no avail. It just seems MTN has a NO CARE attitude towards their customers and they just ignore all queries. We spend hours trying to sort out the account but there is no assistance from MTN. Most of the contracts will be ending in the next 12 months and we will be cancelling all our contracts and moving over to another service provider as MTN does not assist their clients like they used to in the past. Please explain how its possible for this account to vary each month if this is all top up accounts?