Issue

Some phones such as the Yealink T43U have default timer for the screen's backlight. This timer will turn the backlight off after a pre-determined amount of time. 

The backlight of the screen will turn back on when the phone sees activity. Typically this is the user of the phone performing an action such as picking up the handset or pressing a button. But, it also can be activated by BLF keys changing states, such as when another user on the account's phone rings, or when a user goes from being on a call to ending a call. This can be seen as an unwanted side effect. 

Resolve

There are 3 way that the backlight setting can be adjusted to prevent the screen from turning on and off throughout the day. 

1. Set the backlight to Always On. This way the screen backlight will never go dim. 
2. Set the backlight to Always Off. This way the screen backlight will never turn on. 
3. Extend the backlight timer so that it's less likely to turn on and off as often. 

Changing The Setting On The Phone

Depending on the specific device you should be able to find the Backlight Time setting within the menu options of the phone.

If this is a setting that needs to be changed for you entire organziation, please contact support so we can attempt to make this adjustment system wide for you. 

T43U Menu

Menu→Settings→Basic→Display→Backlight→Backlight Time

From here you can adjust the time out value include options for Always On and Always Off.