MTN *****ing data and not honouring contract!!!!!

Once again MTN is *****ing data!!! My contract is 730 each month for 3 GB data and yet for Feb they only credited 2GB. This is the third time I'm complaining about the same thing. I want the outstanding 1 GB as I always pay my account on time and have never skipped a payment in the kore than 20 years I've been a customer. Should I not get the right amount of data from now on, I will simply deduct the amount from the instalment i pay each month. MtN has no right to decide how much data they want to give their clients when a contract clearly states the amount. And they lie on the sms' they send afterwards saying that they sent 3GB when they only sent 2GB!!!!!!