Lumia 720 critical error.

[Hadley_HBK]

When anyone called my Lumia 720 often it says "switched off", but it wasn't. When I realize this, I use to restart (switch -OFF and ON). Then I receive the messages of all missed calls.
:angry::angry:

 

[OzRob]

When anyone called my Lumia 720 often it says "switched off", but it wasn't. When I realize this, I use to restart (switch -OFF and ON). Then I receive the messages of all missed calls.
:angry::angry:

I've seen this sort of issue reported across a number of different phones and platforms and it is usually traced back to a network issue, rather than a specific phone issue. It would seem that certain phones on certain networks in certain areas don't play nicely with each other. Somehow the base station handover or 2G/3G switching goes wrong and the phone isn't registered properly on the network and doesn't switch data packets to the phone correctly. Rebooting the phone reregisters the phone on the network and temporarily solves the issue.

These sorts of issues are notoriously difficult to troubleshoot and solve. The first step would be when you notice the phone isn't receiving calls (which can be hard to do in itself), use another phone to call your service provider and ask them to check whether your mobile is correctly registered on their network. If it is, get them to try to call you phone and see if it rings. If it doesn't they should be able to log an issue and investigate further. They may be able to trace the issue to a setting on their end. Otherwise they may suggest you get your phone checked at a service centre. Either way it's going to be a frustrating process for you, unfortunately.