If anyone's interested, I've get outbound fax semi-working with this setup:

<extension name="faxoutviamitel" continue="true">
<condition field="destination_number" expression="^9(\d{4})$">
<action application="set" data="absolute_codec_string=PCMA"/>
<action application="set" data="fax_enable_t38=true"/>
<action application="set" data="sip_execute_on_image=t38_gateway self 
nocng"/>
<action application="bridge" data="sofia/external/$1@10.10.0.2"/>
</condition>
</extension>

t38modem is started like this:

t38modem -ttt -o /var/log/t38modem.log --no-h323 -u t38modem 
--sip-register t38modem@192.168.20.245,secret --sip-listen udp\$*:6060 
--ptty +/dev/ttyT38-1 --route 
"modem:.*=sip:<dn>@192.168.20.245;OPAL-Force-Fax-Mode=true" --route 
"sip:.*=modem:<dn>"


The remaining problem is that on the receiving end we only have the top 
centimetre or two of the image. I think this may be a tiff library issue 
but I'm not sure.

Cheers

Alex