Avaya VoIP Troubleshooting Expert Needed

I need assistance with an Avaya IP telephone error. The device shows a VoIP network error, and I cannot connect with SIP. I have already restarted the VoIP devices.

Ideal Skills and Experience:
- Expertise in Avaya IP telephony systems
- Strong background in VoIP and SIP protocols
- Experience with troubleshooting network and connectivity issues
- Familiarity with VoIP device configuration and settings

Please provide a detailed plan of action and estimated time for resolution.