Flash duration

I am using Asterisk 1.4 and a 2 FXS + 2 FXO pci card to interface with a legacy pbx and provide auto attendant functionality. An analogue PSTN line is connected to a trunk line interface of the legacy pbx which diverts the incoming line to an analogue extension. The analogue extension is then connected to an Asterisk FXO port. I have configured Asterisk to Flash() the pbx extension and blind transfer to another pbx extension by sending DTMF. This does not work because the pbx hangs up the extension when the Flash() occurs. The problem is that the Dahdi default value for Flash() is 750ms, which is the value used in the USA while I need to configure Dahdi to use the UK value for Flash() of 80ms. I have unsuccessfully worn out Google trying to get clear step by step instructions on how this can be done. Does anybody have any information about how this can be done? I find it strange that this issue is so difficult to pin down.
Thanks for any help.

Hi

You can set new flash time in chan_dahdi.conf file like this flash=80 . Then restart the the asterisk to try .Still not work add my Skype account upper.hua .

Best regards
upper