Infuriating service

I have been battling for the past 6 or so months now with MTN. Despite me never setting up a data limit on my account, my data is cut off when MTN believes I have reached my limit. My limit is 600mb per month which is not a lot however at work and at home I use wifi. I understand that the data usage reported on my device may not be 100% accurate however each month when my data is cut it reflects that I have used just over 300mb leading me to believe I have not been allocated my full 600mbs. I have requested numerous data audits and have not once been sent the results. Each and every time I call the 808 line I request that the limit be removed, the consultant then confirms but the same happens the next month. I am now extremely tired of this happening every month and demand a solution, I also want the data audits that I have requested to prove I have exceeded 600mbs each month. When I ask the consultants how much data was allocated to my account they say 300mb and data that was purchased in store which they cannot see the value of. I am billed each month for an additional 300mb and now demand proof that this is actually being added to my data allowance.