Possible causes for this issue:
No network coverage
IMEI number might be blocked at service provider’s end.
Low signal
Network outage
 Troubleshooting:
Turn off the device and remove SIM card, reinsert SIM card and reboot your phone after 2 to 3 minutes and try making a test call.
Insert your SIM card in different phone, try making/receiving test calls and check whether your calls goes through.
Verify whether your Aeroplane mode is off, when Aeroplane mode is on you cannot transmit or receive wireless transmission.
Contact your Service Provider and check whether your account is updated for services
If issue still persists, try resetting your XP7 to factory data reset by following steps:  Main menu > Settings > Personal > Backup & reset > Factory data reset > Reset phone.
Before you perform this operation please Back up all your contacts, photos, music and other data as this erase all data from phones internal storage.
 

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