I know this is a old post but I have been tinkering with my C811 since I've retired it and found two ways to get in safe mode. At least the phone says it's in safe mode. First way I found here in forums reading old post. With phone on, press the power button to pull up power off- restart menu. Long press the power off option and a pop up ask if you want to reboot into safe mode, select OK. Phone reboots into safe mode.
Second method I found playing with button combinations from phone turned off.
Press and hold volume down and power all the way thru the bootup. Both ways show a safe mode in the lower left of the screen. To exit safe mode, pull up power off-restart menu and select restart.