1. Check the signal strength on the phone screen. To increase signal strength, move around, or if you are calling from inside a building move closer to a window.
  2. Set your phone to use GSM (2G) network only: Select Settings > Phone > Network mode > GSM in case the operator does not permit you to use 3G service (not available on some phones).
  3. Check that your phone is not in offline or flight mode: Select Settings > Profiles.
  4. Restart your phone and make the call again.

If you continuously experience low signal strength in a certain area, network coverage may be insufficient there. For more info, contact your network service provider.