The button with a green handset on it is what you press after you dial a number to connect the call, or to answer the call when the phone rings.
The button with a red handset is to end the call, or drop a call that you don't want to take.
Judging by the other symbol on this button it may also be the power on/power off button.
The Button with a C is normally to move back a level in the menu system, to clear text in a message or delete a digit of a number you have just typed in.
It may also be possible to drop a call by hitting the C button, but Im not sure.
As for when to charge it, one side of the screen will have a battery indicator, probably a line of blocks. If its low, plug it in to charge until the charge indicator stops moving and the battery is full.