1. First of all, check whether you have service.
2. Got service? Then check your remaining calling credit.
3. Still have calling credit? Then switch between 3G/4G/LTE. You can do that in your phone's settings:
- Apple: Settings > Mobile data > select "Enable 4G".
- Samsung (Android): Settings > More settings > Mobile networks > Network mode > select 4G, LTE or an automatic network selection.
- Nokia (Windows Phone 8): Settings > Mobile networks > Highest connection speed > LTE.
4. Are you abroad? Then try selecting another provider. You can do that in your settings as well.