why have limits if you bill past the set limit?!!!

according to my knowledge, I have a soft lock mechanism on my line to stop any further billing on the account when I reach R100 limit. then it is up to me to contact Vodacom to increase the limit temporarily or permanently. I got an sms last week and saying I have exceeded the R100 limit. I then called Vodacom to increase it as I have before if I reach the limit which rarely happens with the wifi router. Now I am told that I have exceeded it by more than R200. my question is how did I get that far? why was the phone not soft locked as you have now when I reached the agreed R100 limit. I am available on my cell between 08:30 and 09:30 or after 18:00. because with the upgrade and this ridiculous extra amount I didn't vouch for, what do you expect me to do? why would I have a R100 limit if I am comfortable with being billed as you please? Please fix this because it is not treating your customer fairly. and I also have upgraded the line and was told it will be a total of R170. I am cringing now in wonder if there are other hidden fees that I should worry about. please lets be transparent and fair.