Overflow Queue

I’m having trouble configuring an overflow queue to work how I want. Here is the issue:

We have 6 phone lines and only 3 reps. Each call takes about 10 minutes. So if all the lines fill up it could be almost 30 minutes before the last person to the enter the queue will get a person.

What I would like to do: I would like that the the first 3 callers enter the queue. Then any caller after that goes to an overflow queue that gets a special message that lets them know the hold time may be longer than normal. I can do all of this. Here is the problem I don’t know how to fix: When both queues have people in them and a caller drops out of the 1st queue there could still be 5 people on hold and the next caller will go into the first queue and not get the message that wait times are high. I thought about setting the overflow queue first and having the calls fail over to the normal queue, but I don’t know what will happen when the normal queue fills up and I don’t want all callers hearing that wait times are high if there are only 3 people in the queue.