Is there a PBX log SIP log file for this? You can find the log by going to Admin->Status->Logfile.
BTW, before you do that, go to Admin->Settings->Logging and setup the following to get the SIP log
1. Log Level - 7
2. Log Length - say 300
3. Log general events, Log SIP events, Log trunk events, Log Other Messages (e.g. INVITE) to "Yes"
4. Save.
5. Go to Admin->Status->Logfile, press 'clear'. This will clear the page.
6. Make the call.
7. Go to Admin->Status->Logfile, press 'reload'
8. Copy the content of the page to notepad file and send it.