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If you want to trace a SIP call, this is what you should do.
When you're at the normal prompt, return to the OAM prompt:
>~.

oam> SIPCallTrace num 0 7 on on

To turn the tracing back off:

oam> SIPCallTrace num 0 7 off off

Useful information can be returned by using:
oam> help sipnpm (for all SIP related commands)
oam> help SIPCallTrace (for specific info on SIPCallTrace)