I noticed some users are being credited with some amount of airtime by MTN but I received some complaints on how to use it, so I decided to share for the benefit of others.

If you have been credited with free airtime by MTN, using the airtime would be impossible until you have updated your biometrics. So, kindly walk into any MTN service outlet and update your fingerprint so you can make use of it. You can check with *559*40#

NB: This is not a cheat. It is a compensation airtime from MTN. Cheers!