Hi Gordon,
I don't believe that parameter is valid for the 9951
Automatically Disabling the Cisco Unified IP Phone Display
To conserve power and ensure the longevity of the phone screen display, you can set the display to turn off when it is not needed.
You can configure settings in Cisco Unified Communications Manager Administration to turn off the display at a designated time on some days and all day on other days. For example, you may choose to turn off the display after business hours on weekdays and all day on Saturdays and Sundays.
To turn on the display any time it is off, press the Select button.
When you turn the display on, it remains on until the phone has remained idle for a designated length of time, then it turns off automatically.
Table 8-1 explains the Cisco Unified Communications Manager Administration fields that control when the display turns on and off. You configure these fields in Cisco Unified Communications Manager Administration in the Product Specific configuration window. (You access this window by choosing Device > Phone from Cisco Unified Communications Manager Administration.)
Table 8-1 Display On and Off Configuration Fields
Field | Description |
|---|
Days Display Not Active | Days that the display does not turn on automatically at the time specified in the Display On Time field. Choose the day or days from the drop-down list. To choose more than one day, Ctrl-click each day that you want. |
Display On Time | Time each day that the display turns on automatically (except on the days specified in the Days Display Not Active field). Enter the time in this field in 24 hour format, where 0:00 is midnight. For example, to automatically turn the display on at 7:00 a.m., (0700), enter 7:00. To turn the display on at 2:00 p.m. (1400), enter 14:00. If this field is blank, the display will automatically turn on at 0:00. The default value is 07:30. |
Display On Duration | Length of time that the display remains on after turning on at the time specified in the Display On Time field. Enter the value in this field in the format hours:minutes. For example, to keep the display on for 4 hours and 30 minutes after it turns on automatically, enter 4:30. If this field is blank, the phone will turn off at the end of the day (0:00). Note  If Display On Time is 0:00 and the display on duration is blank (or 24:00), the display will remain on continuously. The default value is 10:30. |
Display Idle Timeout | Length of time that the phone is idle before the display turns off. Applies only when the display was off as scheduled and was turned on by an end-user (by pressing the Select button on the phone). Enter the value in this field in the format hours:minutes. For example, to turn the display off when the phone is idle for 1 hour and 30 minutes after an end-user turns the display on, enter 1:30. The default value is 1:00. |
http://www.cisco.com/en/US/docs/voice_ip_comm/cuipph/9971_9951_8961/7_1_3/english/admin/guide/9971cus.html#wp1037241
Cheers!
Rob
