Simply performing a hookflash won’t set up a conference call; you need to use the conference feature. Default feature code is 5, yours might be different. From the manual:
Placing Ad Hoc Conference Calls
You can place an Ad Hoc conference call with up to three internal or external parties (for a total of four parties, including yourself).
To place a conference call:
1. While on a call, perform a hookflash, and then dial 5.
2. Place another call, then hookflash and dial 5. (If necessary, repeat this step to place one more call on hold.)
3. Perform a hookflash, and then dial 5 again to join all of the calls together in the
conference.
Dropping Out of a Conference
You can drop out of a conference and return to the conference later.
To drop out of a conference:
Perform a hookflash and hang up. This removes your phone from the conference, but leaves the other parties connected. To return to the conference, lift the handset, and then dial 5.
Ending a Conference and Placing all Parties on Hold
You can end a conference and place all conference parties on Individual Hold, allowing you to toggle between the held parties and speak to one party at a time.
To place a call on hold:
1. Perform a hookflash, and then dial 336.
2. Hang up or place another call.
To return to a call that is on hold:
1. Lift the handset or perform a hookflash.
2. Dial 336.