You are mostly correct.
Once the RAD has started playing, all other callers queue up to the RAD. Then after the RAD hangs up, multiple callers will listen to it at the same time.
I think somewhere around 32 simultanous calls. (been awhile, not sure of the total calls)
What you need to do is install more RADS. How many depends on your traffic and how long your RAD message is. If your RAD is 30 seconds long, if a call misses the start of the RAD cycle, it will wait 30 seconds + call tear down and call setup time.
Ralph