Since the hookflash is just a momentary break in the loop current on the FXS port and cannot be sent as an audio signal, the router passes the hookflash signal via dual tone multifrequency (DTMF) relay as the '!' character. The router with the FXO port then sends a short loop break which the external device sees as a hookflash. To properly pass the hookflash signal, the VoIP dial peers need to be configured for dtmf-relay h245-signal.
The physical port timers have to be adjusted to suit the characteristics of the handset on the FXS port and the duration of the hookflash loop break out of the FXO port as shown below:Check the below link for more inforamtion on how to configure
http://www.cisco.com/en/US/tech/tk652/tk701/technologies_configuration_example09186a008009431b.shtml#procedure1
http://www.cisco.com/warp/customer/788/AVVID/cm_cfg.html