1. Restart your phone.
2. Check the cellular signal strength on the phone screen, and move to an area with better reception.
3. Check that your SIM card is fully inserted. Do not use SIM cards that have been modified to fit the SIM slot.
4. Switch Call forwarding to Off: On the start screen, tap Phone > ... > settings.
5. Remove any kind of protective film or tape from the screen to check if it prevents the touch screen working.