Please refer to the below checklist and follow them properly: 1) The SIM card is inserted correctly (refer to the Quick Setup Guide in User Manual) 2) The SIM card has been activated and has sufficient credit to make calls 3) The phone number you want to call is being input correctly, e.g. with correct area code or country code 4) Make calls at the locations with good mobile network reception 5) They are connected to mobile network and flight Mode is not turned on