[Asterisk-Users] select codec based on extension

[Simone Cittadini]

I've the following installation :

|asterisk client| --- > |asterisk server| --- > |other asterisk server|

all the connections are made in IAX, the client and first server allows 
711 and 729
the other server only allows 729 since it has low bandwidth at disposal

all the numbers but a few are routed to a digium card in the first 
server, the others are routed to the other server, this way :

[default]

exten => _123X.,1,Dial(IAX2/otherserver/${EXTEN})
exten => _123X.,2,Hangup

exten => _X.,1,Dial(Zap/g1/${EXTEN})
exten => _X.,2,Hangup

when I call 123456 from the client box ...

on the client :
Call accepted by asterisk server (format alaw)

on the server :
Call accepted by other asterisk server (format g729)

on the other server :
Called 123456 at something

and then on the server in the middle :
Oct 18 18:00:37 NOTICE[2846]: channel.c:1724 ast_set_write_format: 
Unable to find a path from alaw to g729
Oct 18 18:00:37 NOTICE[2846]: channel.c:1757 ast_set_read_format: Unable 
to find a path from g729 to alaw

since that "something" at the end of the call and the paps which sits 
before the first asterisk server both have g729, I don't like too much 
having to pay to translate something which need not translation.
Is there a clever combination of sip.conf, iax.conf and extensions.conf 
I'm missing to solve my problem ?