Send hook flash to a PSTN line connected through a SIP gateway

Hi all. I know that a hook flash can be send through a dahdi channel but I need to send it through a SIP gateway. I know and verified that the SIP gateway correctly understands DTMF event 16, which corresponds to the hook flash, and is able to send the hook flash to the PSTN line. What I don’t know and this is where I need some help, is how to generate the hookflash from a SIP phone so it gets aent to the gateway. I have read some articles but they all discuss hook flash on dahdi channels. Thanks, Ariel.