Cisco IP phones do not ship with power supply because they are often powered by Ethernet cable through power over Ethernet switch, the power cord is to who do not have access to POE switch will require a the power supply to power the phone. The third generation power supply CP-PWR-CUBE-3= is the replacement of CP-PWR-CUBE-2= and CP-PWR-CUBE= in 2005 as the standard power supply for all Cisco IP phones. The CP-PWR-CUBE-3= is almost identical in appearance to the earlier versions except for the color of the plastic tip around the power plug that is inserted into the back of the phone. Cisco CP-PWR-CUBE-3 is compatible with virtually every phone Cisco makes except for the wireless IP phone, such as CP-792x and some older conference phones such as CP-7935, CP-7936. The first and second generation are not compatible with the newer versions of the 7900 series phones which is why the CUBE-3 is recommended.