Your phone's logs may contain useful troubleshooting information:
SIP signaling and IP packet traces can be collected from the Status menu:
Capturing VoIP, Debug and trace logs:
Under Telephony --> Telephone Numbers the Voice Transmission tab contains information about recent VoIP calls.
Traffic captures can be saved using the following link (and opened using Wireshark): http://fritz.box/html/capture.html
Configuring Debug and syslog Messages from SPA adaptors:
Under the Desktop IP Phone menu choose your model and click the article 'How to Get the Correct Syslog':
The Syslog feature is generally under the Advanced Settings page: