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. Reset your active network connection by switching Flight mode On and back Off (Settings > Flight mode).
  3. 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.