Howto - Only 1 active call at the same time

i have a customer who wants to have configured a Ringall Group or Queue with about 6 Phones.

If 1 call is already answered elsewhere, than noone else should call him anymore until the active call in the queue is done. All other callers should get a busy message or tone.

At the moment, if the call is already answered, the other phones beginn to ring when a second one calls at the same time.

Is there i way i can prevent this?