1) If this issue only occurs in certain places, move to a location with a strong signal and try again.
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 at the top right corner->”Settings”->check “Blocked numbers” & “Caller ID & spam”