Expiration of unused data

I purchased 10gig of mobile data for the month and got 5gig free data that expired within 2 weeks. Instead of using the 5gig that expires first, MTN will use the monthly data so after 2 weeks the 5gigs expires without being used. I queried this on 18/11/2015 but I am still waiting for a reply. They say they are waiting for feedback from their billing department, but it's now been 2 weeks later already. I also had to purchase additional data because of this. My new billing month started 08/12/2015 and exactly the same thing is happening. I contacted them through social media and I phoned their call centre. The reply stays the same, they are waiting for the billing department. Pathetic service.