If you are on the MTN mobile phone network then it is a pretty simple task to check your remaining airtime if you are a prepaid (or even a contract) customer via their nifty little Balance Enquiry feature.
For prepaid, or PayAsYouGo as MTN likes to describe them, subscribers, simply dial 141 from your cellphone to hear an update on your remaining airtime or SMS bundle. Alternatively, dial *141# and MTN will automatically send you a SMS containing your remaining airtime balance.
And how nice of them, both these options are free.
For contract subscribers, a call to 258 will give you an update on your remaining free minutes or SMS bundle. Again, this one is free as well!