A "hookflash" is a brief "on-hook" signal from a handset to a PBX or other switch. This is the same change in voltage that tells the switch that you have hung up the phone, but it is shorter, so that the switch understands that you have sent a "hookflash" instead of actually hanging up. The signal is used to tell the PBX you want to take advantage of some service. This is similar to the way you can quickly press and release the "hang up" button on an older phone to switch between calls if you subscribe to "Call Waiting".
Of course, if there's some VOIP between your handset and the PBX, this change in voltage needs to be converted to some message by your signaling protocol. And, in a lot of equipment, this feature, known as "hookflash relay" is not enabled by default.
While you may not need to resort to the sort of bronze-age "tap the 'hang up' button" signaling if you have a fancy business-grade IP Phone with an LCD screen that shows your calls and plenty of programmable buttons, you may have a classic analog phone deployed somewhere remote and connected into a router's voice module. If this is the case, you may want to enable hookflash relay.
As an example, according to Cisco's website "Hookflash relay is enabled only when the dtmf-relay h245-signal command is configured on the applicable VoIP dial peers." This applies to 2600, 3600 and 7200 series routers. Once enabled, you can configure it by using the "timing hookflash-input" and "timing hookflash-out" IOS commands from the voice port configuration prompt.
Thomas Alexander Lancaster IV is a consultant and author with over ten years experience in the networking industry, focused on Internet infrastructure.