LG P920 User Manual page 394

Troubleshooting
Message
No Sound
Hang up or freeze
128
Possible causes
Vibration or manner mode
Intermittent software problem
Possible Corrective Measures
Check the settings status of the sound menu to make sure
you are not in vibrate or silent mode.
Remove and re-insert battery.
Turn on the phone.
If the phone is locked and does not work, press and hold the
Power/Lock key for 10 seconds to turn it off. Then, press the
Power/Lock key again for 3 seconds to reboot the phone.
Try to do a software update service via the website.