asterisk users - Feb 2008 - How is reinvite triggered

Particularly WRT T.38 fax.

Supposedly, when fax tones are detected, Asterisk is to do a reinvite 
asking for T.38.

Here is what I am using in my dialplan:

[custom-fax1]
exten => s,1,Answer
exten => s,n,StopPlayTones
exten => s,n,Set(FAXFILE=/var/spool/asterisk/fax/${UNIQUEID}.tif)
exten => s,n,rxfax(${FAXFILE})
exten => s,n,Hangup
exten => h,1,system(/var/lib/asterisk/bin/fax-process.pl --to 
${FAX_TO_EMAIL} --from ${FAX_RX_FROM} --subject "Fax from 
${URIENCODE(${CALLERID(number)})} ${URIENCODE(${CALLERID(name)})}" 
--attachment fax_${URIENCODE(${CALLERID(number)})}.pdf --type 
application/pdf --file ${FAXFILE});
exten => h,2,Hangup()