I'm pretty sure I know the answer to this but cannot find it in the documentation and can't get it to work on our demo system but I will ask for confirmation.

Is the Allworx capable of basic monitoring and barging into a call if there is no Queue or ACD feature key installed on the system?

I'm guessing no. Am I wrong?

Try this:

Call Supervision -- Home > Phone System > Handsets > View Handset Preference Group

From the Admin Guide--page 39--Call Supervision: When enabled, the handset can be monitored by another handset that has a Call Supervision PFK

I've never done it---so that's all I know.

I will give that a shot, thanks.

According to tech support all of the Barging and Monitoring functions are possible without Queueing or ACD. Unfortunately the "Whisper" monitor option only works between two 9204's. It's because the 9204's can have two call paths whereas the other models do not have that capability. Which is unfortunate, the Whisper mode is exactly what a customer needs but the supervisor requires a 9224 + a sidecar and the agent at least a 9212. The supervisor needs to inform the agent that they are needed urgently elsewhere and then they can end the call immediately and properly. I can think of alternative means like to have a special extension with Call Monitor that the supervisor can call to let the agent know to respond ASAP. Or the Supervisor can just Barge In.
But I'm not sure if they would want that.

Anyways, some food for thought. I'm sure the Allworx team will change the larger models to be able to use the Whisper feature as well. Hopefully soon.