1. Make sure your phone's profile is not set to silent, beep, or any other profile where alert tones are silenced. 
  2. Use the volume up and volume down keys to adjust the loudness of the alert tones.
  3. Change a different alert tone (Settings > Sounds and vibra > Alert tones). This is to check if the issue is with a specific tone.
  4. Make sure that call forwarding is not switched on (Settings > Calls > Call forwarding).

If the problem persists, remove the battery, and put it back into the phone. Switch your phone on.