Rip-off Full amount charged for pro-rata data

Horrible customer service I have received. I had 2 numbers with MTN, then requested them to make use of one number and transfer data I get from the other number to this one I have had since 2006. This took more than 2 weeks to complete whilst I had paid for this more than a month ago. Once this was done, I got charged the normal contract rate (299)for data But only received 10gig instead of 30gig that I always receive. I was told it pro-rata and no one could advise why the bill is not pro-rata but only the data I have received. To this day MTN has failed me