Snom:
SIP signaling and IP packet traces can be collected from the Status menu:
http://wiki.snom.com/Web_Interface/V8/SIP_Trace
http://wiki.snom.com/Web_Interface/V8/PCAP_Trace
DrayTek:
Capturing VoIP, Debug and trace logs:
http://just.draytek.com/index.php?option=com_k2&view=itemlist&task=category&id=153
FritzBox:
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
Cisco/Linksys:
Configuring Debug and syslog Messages from SPA adaptors:
https://supportforums.cisco.com/document/36921/using-slogsrvexe-utility
Yealink:
Under the Desktop IP Phone menu choose your model and click the article 'How to Get the Correct Syslog':
http://support.yealink.com/faq/faqSearch?p=11
Grandstream:
The Syslog feature is generally under the Advanced Settings page:
http://www.grandstream.com/support/faq/troubleshooting/troubleshooting-questions