I think it can be either the phone or the network. Try a different SIM in your phone to check.
- A simple restart solves the problem, so its unlikely to be physical damage.
- Internet connection remains active, so unlikely to be sim error.
- It is not area or coverage related, because it happens in places e.g. work, where I never have a problem.
- It could be a provider network/admin issue where they temporarily drop your number from the call service and then the phone does not recover until a restart. I would expect more people to get the same error then.
- Combination of phone software and features and apps that are not working together well? I use bluetooth devices (garmin, interphone). Could this cause the problem? But why only now. And why only me?
It appears to be caused by a single event / glitch, but the phone does not auto recover.
I'm surprised that android does not have some BIT or watchdog that can detect and correct an error like this, seeing as though a simple restart corrects it.
Also, I would expect a big warning on all screens if the phone goes into this mode for whatever reason. If you dont try to make a call, you can go days without realising your phone is blocked.