Sounds similar to a fault I have now and again ...
Once in a while my phone which is a Moto G decides to randomly turn on 'Aeroplane mode'
In this mode you it blocks incoming calls. It is a simple fix to simply turn it 'off' and start receiving incoming calls again.
Go into settings and search for this mode to see if it is switched on. Not knowing your settings layout, I cant direct you exactly. Just google your phone model and aeroplane mode and it will be simple task. If it is on just switch it off. Like I say it is worth a try and a lot easier than phoning a helpdesk !