I have checked on other posts said tick turn on the lock screen sound then reboot, but it is only temporary.
I also found out that another situation also cause no sound. That is turn the phone volume to silent or vibrate mode and then reboot. This also cause no sound on phone call after reboot.
It is annoying that I should turn the phone to vibrate mode when I am on duty. If the system crash and reboot itself, I would miss calls.
Any one could help?