asterisk users - Jun 2006 - "I can hear them but they can't hear me" with VoipBuster

Hi;?

When connecting via VoipBuster or VoipStunt, "I can hear them but they
can't hear me"?. This happens with VoipBuster or Voipstunt.
Registration is done correctly.

I thought it could be something related to NAT, but I don't have this
problem when using VoipUser or Asterisk2PSTN, for example.
?
I tried with different codecs: gsm, alaw and ulaw but no change.
?
So, now?I suppose VoipBuster must be blocking something. 
?
My tests were made with calls to several landlines in Portugal, which?are
currently?"free calls".
?
I am using full cone nat at my PIX. 

Can anyone give me an explanation of what may justify it or a possible solution

my sip.conf 

[vpb] 
type=peer 
secret=xxxx 
username=xxxxx 
fromuser=xxxxx 
host=sip1.voipbuster.com 
fromdomain= sip1.voipbuster.com 
insecure=very 
canredirect=no 
disallow=all 
allow=gsm 
allow=ulaw 
nat=yes 
qualify=no 
context=internal 
externip=my.public.ip.address 
localnet=my.local.ip.address /my.local.subnet.mask?

Many thanks!


Rafael