Fax intermittently fails -codec policy

Hi

I have a pbx connected via the sbc, i have one codec policy that allows g729 *  

There are calls and fax that need to use the same realm fax support g711 codec service provider only support g729, is it possible for me to route fax and calls over the same realm using g729 for calls and g711 for fax?

I tried looking at the acl but am abit confused with transcoding

Thanks