asterisk users - Oct 2004 - Asterisk over NetScreen VPN/SIP protocol

Does anyone have Asterisk running over a "policy-based" NetScreen
IPSEC VPN
with NetScreen OS 5.x?  I have Asterisk behind an NS50 on one end and a
Grandstream phone on the other behind a NS 5XP on the other.  The SIP
session appears to set up fine, but no media goes across - no voice, just
silence.  Everything else works.

This worked fine under Screen OS 4.X, but NetScreen added protocol specific
features in ScreenOS 5 that appear to have tripped me up.  Would appreciate
anyone's help who has already worked through this scenario.  

I'm not sure if this is how to ask the question: what protocol does Asterisk
use to transport voice once the sip session is set up?  Is it H.323, RTP, or
are these different aspects of the same thing?  It looks like RTP
encapsulates the voice.  clarification welcome.

Nathan Smith  McKee, Voorhees & Sease, P.L.C.