1) Set the Core Log Level to DEBUG and click the Save button.
2) Edit every SIP profile involved in the issue you are getting logs for, ENABLE SIP TRACE and click the Save button.
Then hover your mouse over the Configuration icon in the toolbar on the top right and click Reload to apply the changes.
3) Go to System->Reports->Network->Network Capture. Then click Capture and reproduce your issue.
See Using Network Capture Filters for more information on how to capture certain packets only.
4) Stop the trace once the issue is reproduced and download the Wireshark trace.
5) Go to Backup - Restore, then click Backup.
6) Select Support Backup as the backup type, then click backup and download the backup.
Send in both the Support Backup (from step #6) and Wireshark trace (from step #4) to support.