Just a quick note for reference.

I managed to successfully configure the phone bridge with FreePBX as a SIP provider for Big Blue Button using this documentation : https://docs.bigbluebutton.org/2.2/customize.html#add-a-phone-number-to-the-conference-bridge . My trunk is connectd to the SIP server running FreePBX and Big Blue Button is registering as a phone extension to receive phone calls and route them to the conference room.

But using a similar architecture with 3CX instead of FreePBX as a provider was failing. Actually, you have to add the AuthID as "auth-username" attribute (see https://freeswitch.org/confluence/display/FREESWITCH/Sofia+Gateway+Authentication+Params )

In Big Blue Button, the profile file would looks like :

  <gateway name="ANY-NAME-FOR-YOUR-PROVIDER">
    <param name="proxy" value="sip.example.net"/>
    <param name="username" value="EXTENSION NUMBER (for instance 100)"/>
    <param name="auth-username" value="THE AUTHID"/>
    <param name="password" value="PASSWORD"/>
    <param name="extension" value="CALLED-NUMBER"/>
    <param name="register" value="true"/>
    <param name="context" value="public"/>

I hope it will help someone stuck with the official documentation.