I have set up a partner once as a calling group, using a third-party voice mail. The auto attendant gave them choices; one was to set an appt. This choice transferred to a calling group of three phones, and they all ring simultaneously if incoming callers choose that option. If all three of them are busy (which happens all the time) the voice mail picks it up, plays a greeting saying they are all busy, and gives them a choice of leaving a message or going back into the "queue". It works, but barely. There is no indication of call volume and the only way someone can busy themselves out of the group is by using a DND button, and once they figured that out, it became a never-ending battle to get them to not use it. I also put a strobe light as an extension in the calling group and put it where the manager could see it, so as long as the light is flashing she knows there are calls waiting in queue.
I think the reason I had to use a third-party voice mail was so I could give the actual calling group extension (I believe it was 71) a mailbox, which allowed incoming callers to move in and out of the group and opt out to leave messages. Again, not the best ACD but we made it work for about $3500, as opposed to $10000 to do properly with a Magix upgrade. The good news is that when they are ready to do the upgrade the new VM will move with it.
If this route has any appeal to you, I can dig out my programming notes; I did it a couple of years ago (so I suppose the fact that they are still using it means it must work somewhat.) It differs from the ASA/DXD options in that there is somewhat more integration between the ASA/DXD handoff of the call and the voice mail, since all functions are done via the voice mail.