This came out in MCD 4.2 SP2 but the commands are different although I think they still work on 5.0.
From the RTC Enter:
StartWS
<make sip call>
StopWS
As bobcheese mentioned it saves in the /vmail directory.
The format is WS_xxxxxx.pcap and you can save up to 20 files of approx 55 megs each.
The "SIP TCPDUMP ON|OFF" command is entered from the ESM interface.
Ralph