If your Cisco Desk phone requires you to connect via Power over Ethernet (PoE) instead of using an SPA power adapter then please follow the instructions below: 

Power over Ethernet (PoE)

1. Connect the Network (only)

2. Using a regular Ethernet cable, your phone can be powered from a Power over Ethernet (PoE) compliant router or switch.
3. Connect the Ethernet cable between the Internet port on the phone and an available port on the in-line power router or switch.

NOTE:  If you are using Power over Ethernet (PoE) then you do not need to connect the AC adapter.
Make sure the Ethernet cable and router or switch is Power over Ethernet (PoE) compliant.