Invoicing for product you haven't got

This is the second time this happend I phoned 808 on the 12 of march to load a gig of data onto one of my contract lines and we have not received it. Yet MTN bill you for it after numerous consultants reassured me that my account will be credited for the gig i didn't receive this happend in November 2015 aswell and i ended up paying for it. How can you be expected to pay for something you haven't got? I phoned in again today as i received a sms tham my account will be suspended. I first spoke to a consultant by the name of Julia who was rude and then just hung up the phone I phoned in again and another consultant told me tha a request to the billing department was never logged this is pathetic!!! I will not pay for something i haven't received and i want this to be sorted!!! I am so fed up with MTN and the way they treat their customers i regret ever taking a contract with MTN