Please help me figure out what I'm missing when setting up a conference

I am following these instructions to set up a new conference:

Here is the information that I’m entering:

Extension 500 is unused in my system.

When I dial my (otherwise working fine) phone system and enter extension 500, it is not recognized. The IVR ignores my input.

Can anyone please point me in the right direction to figure out what I’m missing? It seems so simple but it’s not working.

I’m using FreePBX

Thanks for any help

Is 500 an option in your IVR? It’s not available by checking the box like extensions are. What happens when you dial 500 from an internal extension?