2) Check whether there are smudges or scratches on your SIM card's metal surface as a defective SIM card may affect signal stability. If necessary, contact your operator to obtain a replacement SIM.
3) Check whether “Blocked numbers” or “Filter spam calls” is active on your phone: touch “Phone” app-> touch settings icon
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4) Please try to recovery with turn on and off Airplane mode or reboot the device;
5) Please contact your service provider to check if any re-activate process needed for your service account.