You've activated manual network selection and are out of range of the selected network
Possible cause 5 of 7:
If you've activated manual network selection and are out of range of the selected network, you can't make calls.
What you need to do:
You can set your mobile phone to select a network automatically or you can select a network manually. If you select a network manually, your mobile phone will lose connection to the network when the selected network is out of range.
Select one of the following options:
Select a network manually, go to 2a.
Select a network automatically, go to 2b.
2a - Select a network manually
If your mobile phone has been set to search for networks automatically:
Press the indicator next to "Automatic" until the function is deactivated.
Your mobile phone searches for networks within range.
Press the required network.
Your display will tell you if you have access to the network you've selected.
2b - Select a network automatically
Press the indicator next to "Automatic" until the function is activated.
Return to the Home screen
Press Settings to save the setting.
Press the Home key to end the session and return to the Home screen.