Make sure the "fixup protocol h323 1720" line is in your config.
Make sure you have an access-list that permits that port to your destination.
Earlier versions of the PIX cannot do h323 with PAT, but can with NAT. The should be fixed by your version (6.2), but you may want to try upgrading it regardless. Likewise, you could try doing a static translation instead of PAT if that is what you're doing.
That about taps out my netmeeting knowledge. If this doesn't help, please post your config and maybe someone else will know more.
all the best,
-gbiello