I already checked, those, but I can seem to find a 'waiting in queue' property.
I tried to use Architect's "Position in Queue" variable in this way:
At the beginning set a integer variable to : "Position in Queue + 1", and add a screen pop to show the value in the script.
So when the call is answered by an Agent we should see the remaining positions, which equals the queue size.
There should always be at least a value of 0, but nothing is displayed in the Script.
In the Developer Center, I can only find Estimated Wait Time: