Screen does not always stay on during call

[darkbulb]

I have set the backlight timeout to 15 seconds, so the display turns off after that time.

When the phone is on the idle screen the screen turns off after 15 seconds as expected. Also, as one would expect, the screen doesn't normally turn off while a call is active.

However, in the following scenario, the screen does unexpectedly turn off even though a call is active:

• Call someone
• Transfer to a nonexistent extension (press transfer, some invalid number and hit OK)
• Immediately receive callback from phone system (in my case Asterisk) because of failed transfer
• Answer call
• Wait 15 seconds
• Screen turns off while call is active