Yawming - you have the callflow exactly.
External caller A calls hunt group 5000 (the ACD) - that gets routed to the FXS lines on the PBX and then PLAR'd via the FXO to the SIP extensions. Say 1001 answers on the SIP endpoint. They require to transfer BACK through the FXS-FXO
That's where we need the hookflash out to the FXO, grab dialtone off the pbx, then transfer back into the pbx.
I thought originally we could look for a SIP REFER but they are very capable on the SIP side, and can program a "flash" key if required. I think they can send "*8" or any combination of DTMF to be recognized to trigger the send hookflash to the call let attached to the FXO.
I found an older TCL script that Bryan Deaver put toghether and haven't been back to the customer site yet to test - and I only just got my lab up and working yesterday but my FXO is not recognized.... 
Does that help clear up the call flow - but yes - you are 100% correct on the requirement.
--- this was Bryan Deaver's script that references using DTMF as a reference.
app_hookflash.tcl
#
# written by Bryan Deaver based on the default session application.
# This is an example of converting a * dtmf event into hookflash out the fxo port.
#
# This is written as is with no claims of support. It has been shown to work
# to convert a * h245-alhpanumeric event from callmanager while running on a c2800
# gateway running 12.4(20)T. There are other modifications that would be done
# for example look at * 8 before forwarding the other digits.
# Or converting an A/B/C/D dtmf event into hookflash to interwork with
# other applications. If you find a useful modification, please do drop me a note
# at bdeaver@cisco.com
#
# This is based upon the default session application as seen with
# "show call application voice session" on the router. This provides an
# easy start by which to modify scripts to fit your needs.
#
# The modification of the session script was to add the act_ProcessDTMF function,
# add a section to look for digits coming in from the outgoing call leg
# as well as add an entry to the fsm table.
#
#---------------------------------
# Example Script
#-----------------
proc act_Setup { } {
global dest
global beep
set beep 0
if { [infotag get leg_isdid] } {
set dest [infotag get leg_dnis]
leg proceeding leg_incoming
leg setup $dest callInfo leg_incoming
fsm setstate PLACECALL
} else {
leg setupack leg_incoming
playtone leg_incoming tn_dial
set param(dialPlan) true
leg collectdigits leg_incoming param
}
}
proc act_GotDest { } {
global dest
set status [infotag get evt_status]
if { $status == "cd_004" } {
set dest [infotag get evt_dcdigits]
leg proceeding leg_incoming
leg setup $dest callInfo leg_incoming
} else {
puts "\nCall [infotag get con_all] got event $status collecting destination"
# use invalid-number cause code
leg disconnect leg_incoming -c28
call close
}
}
# here is the process that intercepts the '*' and sends out a hookflash
proc act_ProcessDTMF { } {
# get the dtmf and duration
set dtmf_value [infotag get evt_digit]
set dtmf_duration [infotag get evt_digit_duration]
if {$dtmf_value == "A"} {
leg sendhookflash leg_incoming
} else {
# ok, not a * so playout the dtmf event with the same duration
leg senddigit leg_incoming $dtmf_value -t $dtmf_duration
}
}
proc act_CallSetupDone { } {
global beep
set status [infotag get evt_status]
if { $status == "ls_000"} {
set creditTimeLeft [infotag get leg_settlement_time leg_all]
if { ($creditTimeLeft == "unlimited") ||
($creditTimeLeft == "uninitialized") } {
puts "\n Unlimited Time"
} else {
# start the timer for ...
if { $creditTimeLeft < 10 } {
set beep 1
set delay $creditTimeLeft
} else {
set delay [expr $creditTimeLeft - 10]
}
timer start leg_timer $delay leg_incoming
}
# setup the outgoing leg to monitor for digits
set param(consumeDigit) true
set param(enableReporting) true
leg collectdigits leg_outgoing param
} else {
puts "Call [infotag get con_all] got event $status collecting destination"
call close
}
}
proc act_Timer { } {
global beep
global incoming
global outgoing
set incoming [infotag get leg_incoming]
set outgoing [infotag get leg_outgoing]
if { $beep == 0 } {
#insert a beep ...to the caller
connection destroy con_all
set beep 1
} else {
connection destroy con_all
fsm setstate LASTWARN
}
}
proc act_LastWarn { } {
media play leg_incoming flash:out_of_time.au }
proc act_Destroy { } {
media play leg_incoming flash:beep.au }
proc act_Beeped { } {
global incoming
global outgoing
connection create $incoming $outgoing }
proc act_ConnectedAgain { } {
timer start leg_timer 10 leg_incoming }
proc act_Ignore { } {
# Dummy
puts "Event Capture"
}
proc act_Cleanup { } {
call close
}
#------------------
# State Machine
#----------------------------------
set fsm(any_state,ev_disconnected) "act_Cleanup same_state"
set fsm(any_state,ev_digit_end) "act_ProcessDTMF same_state"
set fsm(CALL_INIT,ev_setup_indication) "act_Setup GETDEST"
set fsm(GETDEST,ev_collectdigits_done) "act_GotDest PLACECALL"
set fsm(PLACECALL,ev_setup_done) "act_CallSetupDone CALLACTIVE"
set fsm(CALLACTIVE,ev_leg_timer) "act_Timer INSERTBEEP"
set fsm(INSERTBEEP,ev_destroy_done) "act_Destroy same_state"
set fsm(INSERTBEEP,ev_media_done) "act_Beeped same_state"
set fsm(INSERTBEEP,ev_create_done) "act_ConnectedAgain CALLACTIVE"
set fsm(LASTWARN,ev_destroy_done) "act_LastWarn CALLDISCONNECT"
set fsm(CALLACTIVE,ev_disconnected) "act_Cleanup CALLDISCONNECT"
set fsm(CALLDISCONNECT,ev_disconnected) "act_Cleanup same_state"
set fsm(CALLDISCONNECT,ev_media_done) "act_Cleanup same_state"
set fsm(CALLDISCONNECT,ev_disconnect_done) "act_Cleanup same_state"
set fsm(CALLDISCONNECT,ev_leg_timer) "act_Cleanup same_state"
fsm define fsm CALL_INIT
