Mtn has introduced Free missed call alert whist your device is switched of.
This means that even if your device is switched off, when you put it on, you will get notification of those that tried calling your line.
According to mtn
Y'ello! You can now receive alerts for all calls missed when your phone is turned off or out of coverage area. Send MCN to 131 and activate. Its Free!
HOW TO GET MISSED CALL ALERTS ON MTN
To get missed call alerts even when your phone is switched off, sms MCN to 131.
It's free according to the text MTN sent.