A Yealink desk phone sidecar stuck in a reboot loop can be frustrating, but there are several troubleshooting steps you can try to resolve the issue:

Power Cycle the Sidecar:

• Disconnect the sidecar from the main phone.
• Unplug the power cable from the sidecar.
• Wait for one minute, then plug the power cable back in.
• Reconnect the sidecar to the main phone.

Check Cabling and Connections:

• Ensure that all cables connecting the sidecar to the main phone are secure and not damaged. Check both ends. A loose or damaged cable can cause communication issues.

Test with Another Phone:

• If possible, test the sidecar on another Yealink phone of the same model. This can help determine if the issue is with the sidecar itself or the phone it's connected to.

Check Power Source:

• Ensure that the sidecar is receiving power from a stable source. If the power source is unstable, it could lead to rebooting issues. Consider plugging it into a different power outlet or using a power conditioner if available.
• Some sidecars require their own power source while others can be powered by their connection with a phone. 
• If you are daisy chaining multiple side cars together, it typically require extra power. 

Network Configuration:

• Check if there have been recent changes to your network configuration. Sometimes, network issues can affect the proper functioning of devices. Ensure the phone and sidecar have a stable network connection.

Contact Primecall Support:

• If none of the above steps resolves the issue, it's a good idea to contact Primecall customer support or your IT department for further assistance. They may have specific troubleshooting steps for your model or be able to provide warranty support if the sidecar is still under warranty.

Remember that troubleshooting may vary depending on the specific model of your Yealink desk phone and sidecar. Always refer to the user manual and official Yealink resources for guidance specific to your equipment.