Well, first you need to decide how you want to use the phone and thus what the "Line mode" is set to. By default it is "session mode" which means only the 5 buttons on the left can be used for lines/speed dials/BLFs/functions/etc. and the buttons on the right are used to manage calls. You can make the right side buttons usable for the same functions as left by changing the line mode to "Enhanced line mode", but then you lose the ability to easily toggle/manage the calls. As to what the buttons need to be it depends on your individual user requirements, i.e. if the user has only 1 line and nothing else then having those assigned as speed dials or BLFs is usually good solution, otherwise you assign the buttons to what is needed, i.e. other lines, intercom, etc.
