Help Unregistered SIM

[bigrigfixer]

This has happened to me a couple times now. Usually find out about it because of delivered sms. Also, from the lock screen it'll say "unregistered SIM".

So far, restarting the phone solves the problem. But I'd like to know what is causing it?

 

[FxsX24]

I also get this issue from time to time

 

[bigyid]

Same