Mitelpassion, my best explaination will come from a recent experience.
One of my customers required a Pre-announce to their ACD queue. Numerous factors limited my options, not the least of which was prairiefyre reporting and Utility oversite (Company is a natural gas provider). Basically I had to keep all the numbering the same.
My solution was to create an Agent group with no agents as the primary group and set the overflow to the actual group after the initial RAD had delivered the preannounce. Worked like a charm and if the overflow worked the way you thought (or I used to think) there would be no problem.
The problem was that calls were getting stuck in queue (Reader boards displays calls in queue) but even with agents available, the call would not deliver.
I duplicated the problem by making all agents busy at the time of overflow. The call only tries to overflow the 1 time and having no agent available, stayed queued to the group with no agents and never tried again.
I have yet to come up with a decent solution to this issue. My work around for the time being is to have a special agent code that can be used to log into the Special Group" to retrieve the call.
*******************************************************
Occam's Razor - All things being equal, the simplest solution is the right one.