Transcoding question

Hi everyone. I have a little question about the transcoding. I have a Klarya Openvoice PBX equipped with asterisk 1.2.X that uses snom 300 phones. The phones are configured with this codec order:

Codecs : 0x104 (ulaw|g729)
Codec Order : (ulaw,g729)

I need this codec order for certain features.

I need also a g729 only IAX channel for the outgoing traffic. The problem is: when i make a call from a phone on this IAX channel, the call format chosen is ulaw transcoded to g729, and this is a useless waste of a g729 license.

So the question: mantaining this codec order, is it possible to force the call to be established in g729 pass-through mode?

Thanks everyone in advance and excuse me for the bad english!