Make sure that your device is not muted and the volume level is sufficiently high. If you still can't hear anything, you have a problem with the speakers. Refer to the troubleshooting page for further information on how to replace the speakers.
This problem might be due to dead batteries or a problem with the connection between the telephone and wall jack. Make sure you check that both ends of the phone line are securely plugged in the phone-jack in the wall and in the handset. If you still have a problem, you might have to replace the screen or contacting your provider.