overbilling

I have a 2Gb/R99 data bundle with mtn. August i was charged R1700 for apparently goiny out of bundle, and in October R3000. Monitoring my dat i used 837mb and 300mb respectively in those months. After 8 phone calls and 4 reference numbers later, i'm still stuck without answers. And the worst is THEY CAN'T PROVE THE USAGE. They've promised to send a breakdown (3 times) and i'm still waiting. Hpw bad can service be?!