Making 3 way call on carrier's trunk with flash hook with DAHDI

I have analog DAHDI ports. The phone lines are analog, from Comcast. I am trying to write a dialplan that will –

  1. Dial one person
  2. After the call is connected to Person 1, then send a Flash Hook, and get a second dialtone. Then, dial another number.

This is how 3-way calling works on the carrier’s trunks. I want the 3-way calling to be done on the carrier’s trunks at the Central Office instead of in the Asterisk machine.

Does anybody have any sample code that does this?

I’m thinking that to do the flash hook, I should be able to use SendDTMF(f) however this doesn’t seem to be working. The flash hook duration seems to be hard-coded in the DAHDI program at 750 milliseconds. There doesn’t seem to be a way to override it with a config setting.