Core Issue
When VoIP technology is integrated to legacy PBX and Public Switched Telephone Networks (PSTNs), there is sometimes a need to pass a type of signaling known as hookflash. A hookflash is a brief interruption in the loop current on loopstart trunks that the attached system does not interpret as a call disconnect.
Resolution
Yes, Callmanager Supports HookFlash and the Cisco CallManager interpretation of the hookflash is to invoke supplementary features for call transfer. This is enabled by default on the CiscoCallManager. The interpretation of the hookflash cannot be changed to represent other features, like conference or three-way call.
For more information, refer to Hookflash Transfer in MGCP. The document focuses on how to configure and troubleshoot hookflash transfer on the analog gateway with Cisco CallManager and the Media Gateway Control Protocol (MGCP) signaling protocol.
We can also increase the hookflash timer using timing hookflash-out msec command .For information on hookflash configurations on Foreign Exchange Station (FXS)/Foreign Exchange Office (FXO) ports, refer toConfiguring Hookflash Relay on FXS/FXO Voice Ports .
