Overcharging

I have a R100 call limit on my Smart S package. Please explain how is it possible for my May bill to reach R2500. Only to find out after 5 years that the limit we put and obviously what we can afford is not guaranteed. When ever I've reached my limit my line gets soft locked but in this instance it gets soft locked after reaching R2000. Is this your way of making easy money. Every 8/10 customers experience the same problem.