asterisk users - May 2008 - chan_sip Maximum retries exceeded on transmission

I have a situation here where a user has an AAstra 480i phone, which 
function corectly. The phone is behing a nat-router (a linksys wrv200 for 
it's VPN point to point facility). The phone is plugued in a port wich has 
qos enabled.

And when the user places a call, sometimes (not always), we get this in the 
console :

[May  8 13:41:55] WARNING[5804]: chan_sip.c:1948 retrans_pkt: Maximum 
retries exceeded on transmission 
4d44e080d246b7d93d4cd7e2ec50ef64 at 66.131.168.221 for seqno 1867350652 
(Critical Response)
[May  8 13:41:55] WARNING[5804]: chan_sip.c:1972 retrans_pkt: Hanging up 
call 4d44e080d246b7d93d4cd7e2ec50ef64 at 66.131.168.221 - no reply to our 
critical packet.

And then, the call stops.

I did enabled sip debug for that peer, and I saw that a packet was sent to 
the phone, and the phone did reply (I see it in the debug).

In sip.conf, for that peer, I have nat=yes.

Any ideas ?