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Firstly... All you have to do is to dail this no "*406#" to be part of MTN I_PLUSE but if you already on MTN I_PLUSE don't worry about the first step.
Secondly... Anytime you want to recharge on your phone just dail this the same number*406# before you load the recharge card, then you will see that Mtn has just given you 10Mb free. That all.