1. Make sure that the Ringer volume is not set too low. On the Start screen, press volume up key several times to see the number increases in the upper left corner.
2. Check if the issue is with a specific ringtone: Go to Settings > ringtones+sounds. Under Ringtone, open a list and choose an item. Tap on a play button to preview the ringtone.
3. Make sure that call forwarding is switched off: On the Start screen, tap Phone > ... > settings and switch Call forwarding to Off.
4. Connect a compatible wired headphones. If the headphones work, disconnect them, restart your phone and continue troubleshooting the internal speakers.