Normally, once your phone is switched off, you cannot know what activities are going on, in your line.
But with this helpful tip from mtn, you can now receive free missed call alert once you switch your device ON again.
•How to Activate mtn Missed call alert
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•Send MCN to 131
• You will receive a notification from Mtn that you have successfully be activated for free
• To unsubscribe, kindly dial NOMCN to 131.
And you will be removed from the service.
I hope this helps.