asterisk users - Sep 2004 - Queues : Rings even when the agent is on a call

I am using asterisk in a small call center where agents not only receive
calls but also call outside themselves. The queues work fine. However,
when an agent has called outside and is already on the phone, then also
he gets ACD calls. Is there a way to stop ACD calls to agents who are
already off hook.

Thanks,
-- sudhir