Many people are complaining that after the expiration of their previous MTN internet data subscription, and that when the subscribe again that their unused data is not being added to their new subscription as stated by MTN.

The simple reason why your unused data is not being rolled over to your next subscription is because you are not playing according to MTN rules of data rollover. If you want your unused data to be rolled over into your next subscription, simply make sure that before the expiration of your active data plan, you have recharged your account balance.

By so doing when your active data expires, you will automatically be subscribed into another plan and your previous unused data will be added to the new subscription.

http://highteknology..com/2014/07/how-to-roll-over-your-mtn-unused.html