Please check the mute button on your device and set the volume to maximum.
Still no luck? Here's some other things you can try:
- Reboot the phone (for instructions: http://support.apple.com/kb/ht1430). Then see if it works.
- Check if the Do Not Disturb mode is disabled.
If it doesn't:
- Try a headset if you have it.
If that doesn't help, you can try:
- Updating iOS: http://support.apple.com/kb/ht4623
- Try the instructions in this video: