It's <Mute and Forward> on the 5000. Maybe they use Left and Right because you can't put little arrows on the hard keys. I agree it's a waste of limited buttons and I always remove them.
Hmmm... Unfortunately our test bench 5000 is in another location, all I have access to is IP phones here.
The user guide for 8528 and 8568 phones says:
Hold - Left/Backspace when entering dialpad characters
Transfer - Right/Forward when entering dialpad characters
Message - Toggles between Alpha Mode and Numeric Mode when entering dialpad characters
Can anyone confirm this? I have about 50+ Desi labels I need to print, and would really like to re-assign these buttons to other functions.