The Cisco Unified IP Phone 7931 includes 24 buttons that can be assigned to lines and call features. A 3-color LED provides call status information for each line.
The line buttons are numbered 24-1 from top to bottom. The numbers do not appear on the phone.
There are two ways how the top four buttons can be programmed.
ephone template 15
button-layout set phone-type [1 | 2]**
** Specifies which fixed set of feature buttons appears on a Cisco Unified IP Phone 7931G that uses a template in which this is configured.
•1—Includes two predefined feature buttons: button 24 is Menu and button 23 is Headset.
•2—Includes four predefined feature buttons: button 24 is Menu; button 23 is Headset; button 22 is Directories; and button 21 is Messages
More on 7931 and CME here.