[Asterisk-Users] ATA not sending data to asterisk?

[Matt]

Hi,
I've just setup a second asterisk system and am having a wierd
problem.   If I am on the same network as the system it works fine...
if I am outside coming in from the internet through a NAT I get the
following:

I can place calls.... I can hear the asterisk system.  The asterisk
system does not seem to hear me.  When I do a tcpdump I don't even see
the ATA sending anything to the asterisk box (when I tcpdump on the
network side of the ATA).   I've tried 2 ATAs, and X-Lite on a mac and
windows 2000.. all exibit the same issue on this setup.   Any ideas?

What would cause the ATA and Softphone to not send data (UDP streams)
to the asterisk box?