After logging-in into the phone web interface:
  1. Click on the PCAP Trace link
  2. Click on the Start button
  3. Reproduce the issue
  4. Click on the Stop button
  5. Download the trace

Please remember: the PCAP trace phone buffer is limited to 500 KB, in case the PCAP trace exceeds this value the downloaded trace will contains only the last 500 KB.