Wrong code has been sent before, here is the right one for sofia:
<extension name="att_xfer">
<condition field="destination_number" expression="^att_xfer$">
<action application="read" data="1 15 /opt/freeswitch/sounds/en/us/callie/misc/8000/transfer1.wav callednumber 7000 #"/>
<action application="att_xfer" data="sofia/${domain}/${callednumber}"/>
</condition>
</extension>