Set a phone receive only one call

Hi,
all my phone are in a queue and can receive more call in the same time (this phones must remain so).
But i’ve a phone not in the queue that must receive only one call, the second call must receive the busy signal. I’ve setup Call Waiting= disable. In this case i don’t receive the call waiting signal but the caller receive the free signal for all the setting ring time and after the voice say the line is busy…
If i disable the call waiting the second call must receive the busy signal.
Is a bug?

Asterisk 1.6.1
Freepbx 2.5.1.5

Check to see if there is a follow-me set up for that extension - if there is, that may be part of the issue - and also make sure you don’t have voicemail enabled.