asterisk users - May 2005 - sip phone behind nat connecting to an asterisk box that has one port on the open internet

Hello all,
  I have an asterisk box on the open internet (one port). I can get
sip clients from behind a nated network to register with this box and
make phone calls, but when the other person picks up they can hear me
just fine but I cannot hear them at all.  Is there anyone that can
help me get this working.  Right now I am open to any type of sip
phones soft or hard to work so I can then narrow it down from there. 
The reason for this is that I have a friend that has a mac who needs
to connect to this asterisk box and the iaxcom application that speaks
IAX is a little bit unstable on the mac.  I guess if someone knows of
a good IAX soft phone for mac then that would be another option.

-- 
Thanks,

Lance Grover