Try
Router(config)# telephony-service
Router(config-telephony)# service phone daysBacklightNotActive 5,6
Router(config-telephony)# service phone backlightOnDuration 01:30
Router(config-telephony)# service phone backlightIdleTimeout 01:00
Router(config-telephony)# create cnf-files
Router(config-telephony)# reset all
Also
What firmware version is on the phone?
What version of IOS/CME?