asterisk users - Apr 2007 - Pass-thru

Hi all,

Here is my configuration: Phone ?? Asterisk ?? Gateway (SIP 2 PSTN) 
In the Gateway (patton) I have in "codec order" G729 then G711 

If the Phone use G729, I have a pass-thru in the Asterisk ... It's the main
case.
But If I put G711 in the Phone, I want that the Asterisk try a G711 codec with
the Gateway !! because in this case, it's G711 between the Phone and the
Asterisk and G729 between the Asterisk and the Gateway.

Do you know if I can do something like this?

Thanks a lot for your help,

Thomas