You are signed in to your Polycom CX600 through your Unified Communications account but your phone is not connected to your computer. It is likely plugged into the power, but not physically plugged into the computer.

Find the cord that is used to plug the phone into the computer via USB. Plug the system in. There may need to be further configuration done in Skype for Business to set the handset as your default input and output audio. If you are not able to find a cord, contact your local IT group to see if they have a spare.

The phone will still work to make basic outgoing calls and receive calls. However, you may be missing out on functionality available to you through Skype for Business.