tcpdump is a Linux utility for capturing and analyzing network traffic. It allows you to monitor packets passing through a network interface in real time and save them for further analysis.
Connect to your PBX via SSH (instructions here).
Run the following command:
Reproduce the issue by making a phone call. After the call, press CTRL + C in the SSH console to stop tcpdump.
Connect to the PBX using WinSCP (instructions here).
Send the log file /tmp/capturefilename.pcap to technical support.