I'm not sure I have the full terminology to explain this as a Mitel trained technician would be able to..
We have a PRI connected to the PBX and have multiple different numbers for that PRI. The PBX recognizes one specific number for the ACD (which is our incoming 1-800 number). When that number is an incoming call it directs it to an initial announcement that plays, then immediately is pushed to the ACD programming/announcements. There the call waits for an available agent.
I tested this issue with my own phone, I logged into the ACD. Called in on my cellphone, took the call from my cellphone with my ACD logged in. Then I placed the call on hold by pressing the hold button and hung up my handset. (I was using a 5320, but Agents have 8568s)
Two things happen while an ACD call (that has been answered by an agent) is placed that on hold. First there is a periodic display and a short ring on the phone showing how many callers are waiting in the ACD. Secondly if your phone is selected by the 'longest idle' logic in the ACD/Hunt-Group configuration, your phone will start ringing on Call Key 2, display the Caller ID of the incoming call. So this isn't just that (I think it's called?) Camp-On Notifications showing that they're seeing. I did test this and had an ACD call directed to my Extn/ACD login while the first call I accepted from the ACD was holding (me on my cellphone).
Does that answer your question?
Thank you,