How Can I Subscribe To MTN 50MB For 50 Naira Data Plan?
The 50MB for 50 naira data plan last for 7days and it offers you 50MB plus 5 minutes of call time for making calls.
To activate it, make sure you have at least 50 naira on your MTN SIM, then migrate to MTN BetterTalk plan by dialing *123*2*6# or send BT to 131 in an SMS. Next, go to your text message box and send J12 to 142 and you will immediately receive a confirmation message or SMS that you have successfully subscribed for this plan
Source: http://360tech.tk/2016/05/how-to-get-100mb-for-n100-and-50mb-for.html?m=1