In order to better troubleshoot issues with your SPA ATA or SPA IP Phone, you can capture all network traffic off of the
network cable using Wireshark.
Wireshark is extremely capable and very powerful. This document only provides the bare minimum information to help get you started.
Important security notice.
Catched data may reveal some sensitive data including usernames and password unrelated to SIP communication. Take it into consideration before you will share Wireshark's output with a stranger.