asterisk users - Jul 2006 - how to send flash command from asterisk to old pbx when pressing button on phone

Hi,
I connected an Asterisk box to an old pbx using a TDM400P (one fxs and 
one fxo). Then I connected an analog phone to Asterisk FXS port. Is it 
possible to send a flash command to old pbx via asterisk box when 
pressing flash button on the analog phone?
When I press the flash button the console shows asterisk putting the 
call in hold:

    -- Starting simple switch on 'Zap/3-1'
    -- Executing Dial("Zap/3-1", "SIP/linux") in new stack
    -- Called linux
    -- SIP/linux-739c is ringing
    -- SIP/linux-739c answered Zap/3-1

When pressing flash button:
    -- Starting simple switch on 'Zap/3-2'
    -- Started three way call on channel 3
    -- Started music on hold, class 'default', on channel
'SIP/linux-739c'
    -- Stopped music on hold on SIP/linux-739c

This point I hear a strange sound.
Then pressing the flash button again:
    -- Dumping incomplete call on on Zap/3-1
    -- Hungup 'Zap/3-2'

I'd like asterisk send the flash command to the old pbx. The flash 
command does not work.
Is there anybody who could interface asterisk and a normal pbx so that 
the users could use the same phones in the same way?

TIA

Giorgio Incantalupo