Well this is something some people may have been careful enough to notice and courageous enough to try. It is not something so complex but rather very simple and I will say commendable. To triple your recharges on MTN, simply buy the recharge card, scratch it to unveil the pin the load with this code *888*PIN# instead of the usual *555*PIN#. You will receive a notification confirming your recharge with additional twice the amount recharge. This is the simple mathematics, load N200 airtime and get N600 worth of airtime to call your friends and loved ones. Enjoy.
http://www.xtrabrainz.com/2014/12/how-to-triple-your-mtn-recharge-credits.html

Abeg forget MTN

don't follow the process When you are sent by Oga or Madam to buy airtime because you will not be able to transfer the main balance in fact you may not be able to view it but indeed it exist. also I think it is given only in weekend.