This guide details the steps to take off the rear cover of your Inter-tel IP 8660.
Before disassembling the phone, make sure it is not plugged into power and the handset phone cord is disconnected from the handset jack.
Unscrew the 9 highlighted screws using a #1 Phillips head screwdriver. You can use a magnetized screwdriver to draw them out.
8 mm long
11 mm long
Lift up the back cover and set it aside.