That FAC is for analog/SL devices... Check the help for the feature Conference and you will see that the 5212 doesn't need a FAC to do a conference. Just TRANS/CONF dial next number, T/C and dial next party, T/C dial next party, and T/C twice to join them all together.
Yup, all of my testing had been done while checking the help section for the conference page. What led me to create this post was the fact that we need conference calls to work for soft phones, not necessarily for 5212 IP phones. I thought "if the FAC doesn't even work on the 5212, how could it even work on the soft phone". But I did learn something with your post, i can have more than 3 party members in a conference by following what you suggested, thank you for that!
We're testing out the 3cx soft phone (no server installed, just the soft phone connected to the pbx) and our users are wondering if call conference works. For some of them, it would be a deal breaker, to switch to a soft phone.