ROUND ROBIN call auto transfer if busy or on phone?

We choose round robin with AGENTS CAN OPT OUT OF CALLS OFF. This keeps those who "just don't want to take calls" from doing so. But if they are on a call the phone keeps ringing for the AGENT ALERT TIME (ours 35sec) which is not good for the customer. It would be great to have round robin SKIP over that person who is on the call and go to the next one. It would be great to have more settings for these things. ANY IDEAS BESIDES ALL OR NOTHING?