After connecting a Poly Sidecar or Expansion Module to the physical device, the sidecar boots up continuously. The desk phone is connected and working properly, yet the sidecar does not come up with the lines configured in Admin Console. Rebooting the desk phone and sidecar does not fix the issue.

Replace the desk phone the sidecar is connected to, as the AUX port might be faulty.

This is due to a faulty or defective AUX port on the desk phone.