This is how I solved this problem.
Reset the network settings
Hard iPhone reset (press volume up once, volume down once and hold power button)
Then remove the SIM card
Use the phone until it turns off
Put the SIM card back in while it is turned off
Put the phone on charge
Let it stand there till it boots up
I hope this works out for you, I tried everything and nothing helped until I found this solution and it works, at least for me ...
RimoBro
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This is how I solved this problem.
Reset the network settings
Hard iPhone reset (press volume up once, volume down once and hold power button)
Then remove the SIM card
Use the phone until it turns off
Replace the SIM card while it is turned off
Put the phone on charge
Let it stand there till it boots up
I hope this works out for you, I tried everything and nothing helped until I found this solution and it works, at least for me ...